PHP Bible
Chapter 6: Control and Functions
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Summary
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Creating and combining logical tests
Branching with if and switch
Looping with while and for
Aborting page execution with exit and die
Using built-in PHP functions
Exploring the online function documentation
Defining your own functions
Advanced tricks: variable arguments, call-by-reference, variable
functions
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Intro to control

In order to have a program which responds to something (time
of day, user input, or the contents of a database, etc.),
programming languages require a control structure which will
allow branching and looping within an application
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Branches are forks in the execution of your code where you can
go left or right depending on the circumstances at the execution of
the branch
Loops allow a statement or a block of statements to execute
multiple times until a terminating condition is met
PHP's control structure is practically equivalent to C's structure
(and consequently very similar to Perl, Java, JavaScript, etc.)
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Boolean expressions
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Most control structures have 2 distinct parts, the test (which
determines which part of the rest of the structure executes) and
the dependent code
Logical operators combine other logical (e.g. boolean) values to
produce new boolean values. Standard logical operations (and,
or, not, and exclusive-or) are supported by PHP
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and: true if and only if both of its arguments are true
or: true if either (or both) of its arguments are true
!: Not operator - true if its single operand (to its immediate right)
is false, and false if its operand is true
xor: true if either (but not both) of its arguments are true
&&: same as and, but binds its arguments more tightly
||: same as or, but binds its arguments more tightly
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Logical operator precedence
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Just as with the arithmetic operators, some logical operators have higher
execution precedence than others and precedence can be overridden by
grouping subexpressions together using parentheses
In declining order of precedence, operations are performed on !, &&, ||,
and, xor, and finally or
One very handy feature of boolean operations is that they associate left-toright and they short-circuit, unnecessary operations (meaning they do not
even evaluate their second argument if their truth value is unambiguous from
their first argument)

E.g. the statement,
if ($denom && ($numer/$denom > 2)) print "OK";
will not cause an error even if $denom is equal to 0 since in
that case, the test ($numer/$denom > 2) would not
execute since the "anded" operation's outcome has already
been determined since $denom evaluates to FALSE
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Comparison operators
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Comparison operators can be used for either numbers or strings,
however caution should be utilized in comparing anything that
is not an integer
Operator
Name
Behavior
==
Is equal to
True if its operands are equal to each other
!=
Not equal
False if its arguments are equal to each other
<
Less than
True if the left-hand operand is less than the right-hand operand
>
Greater than
True if the left-hand operand is greater than the right-hand
operand
<=
Less than or
equal to
Duh
>=
Greater than or
equal to
Double-duh
===
Identical
True if its arguments are equal to each other and of the same
type, and otherwise false
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Comparison operators (cont.)
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Although over-reliance on precedence rules in comparison,
boolean, and arithmetic operators can be confusing for the
person who reads your code next, it's important to note that
comparison operators have a higher precedence than boolean
operators
Comparing doubles (real numbers) can be problematic when
testing for equality because rounding errors may make 2 values
that are theoretically equal differ slightly

When comparing doubles for "equality", you should test whether
a number falls within an acceptable range using <= and >=
Comparing strings using the comparison operators can also be
tricky since strings will be converted to numbers if possible to
perform the comparison which may yield unexpected results
and comparisons are done on a case-sensitive basis using the
ASCII values for determining order (e.g. 'B' < 'a')
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Branching
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In PHP there are 2 main structures for branching
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if branches are the workhorse in most programming languages and is usually the
first conditional structure anyone learns
switch is a useful alternative for certain situations where you want multiple
possible branches based on a single value and where a series of if statements
would be cumbersome
The syntax for the if statement is:
if (test)
statement-1;
elseif (test)
statement-2;
else
statement-3;

When an if statement is processed, the test expression is evaluated, and the
result is interpreted as a boolean value. If test is true, statement-1 is executed. If
test is false, the next elseif statement is interpreted if it exists. If all if and
elseif clauses' tests are false, then the statement associated with the else
clause is executed if it exists
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IF (cont.)

How does PHP determine which if statement an elseif or
else statement goes with?
if (num % 2 == 0)
if ($num > 2)
print ('num is not prime<BR>');
else
print('num is odd<BR>');

Each else and elseif is matched with the nearest unmatched
if that can be found, while respecting the boundaries of braces. If
you want to make sure that an if statement stays solo and does
not get matched to an else, wrap it up in braces
if (num % 2 == 0)
{
if ($num > 2)
print ('num is not prime<BR>');
}
else
print('num is odd<BR>');
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Switch
For a specific kind of multiway branching, the switch construct can be useful.
Rather than branch on arbitrary logical expressions, switch takes different paths
according to the value of a single expression (usually a variable's value)

switch ($day)
{
case 5:
print('Five golden rings<BR>');
break;
case 4:
print('Four calling birds<BR>');
break;
case 3:
print('Three French hens<BR>');
break;
case 2:
print('Two turtledoves<BR>');
break;
case 1:
print('A partridge in a '.
'pear tree.<BR>');
break;
default:
print('<B>Unknown day</B><BR>');
}
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if ($day == 5)
print('Five golden rings<BR>');
elseif ($day == 4)
print('Four calling birds<BR>');
elseif ($day == 3)
print('Three French hens<BR>');
elseif ($day == 2)
print('Two turtledoves<BR>');
elseif ($day == 1)
print('A partridge in a '.
'pear tree.<BR>');
else
print('<B>Unknown day</B><BR>');
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Switch (cont.)
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The expression can be a variable or any other kind of expression as long as it
evaluates to a scalar value (integer, double, or string)
The construct executes by evaluating the expression and then testing the
result for equality against each case value
As soon as a match is found, subsequent statements are executed in sequence
until the break statement or the end of the switch statement is reached
A special default tag can be used at the end which will match the
expression if no other case has matched it so far (analogous to the else
clause in an if statement)
Multiple values can be specified for an executable block by having multiple
case statements preceding that block of statements
switch (strtolower($up_in_the_air))
{
case 'bird':
case 'plane':
print ('It\'s Superman<BR>');
break;
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Looping
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Bounded loops vs. unbounded loops
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A bounded loop is a loop that will executed a fixed number of
times - you can tell by looking at the code how many times the
loop will iterate, and the language guarantees that it won't loop
more times than that
An unbounded loop is one that repeats until some condition
becomes true (or false) and that condition is dependent on the
action of the code within the loop
PHP doesn't have any bounded loop constructs, but an unbounded
loop can do anything a bounded loop can do
PHP also provides looping constructs which iterate over the
contents of arrays which will be discussed in a later chapter
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While loops
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The simplest PHP looping construct is while, which has the
following syntax:
while (condition)
statement;
The while loop evaluates the condition expression as a
boolean – if it’s true, it executes statement and then starts again
by evaluating condition and repeats this process until condition
is false
 Statement may be a single statement or it may be a braceenclosed ({}) block
 It’s possible that a while loop may not execute even once if
condition is false upon reaching the while loop, or the will
loop may continue executing indefinitely (or until the computer
executing the script runs out of memory) if the condition never
changes from being true
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While loop examples
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The following loop will never execute
$count = 1;
while ($count > 10)
print ($count);
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This loop will never stop executing (infinite loop)
$count = 1;
while ($count < 10)
print ($count);

This is probably what you wanted the loop to do
$count = 1;
while ($count < 10)
{
print ($count);
$count++; // same as $count = $count + 1;
}
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Do-while
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The do-while loop is similar to the while loop except that the
test is performed at the end of the loop rather than the beginning
and consequently the loop will always execute at least one time
$count = 45;
do
{
print ("count is $count<BR>\n");
$count++;
}
while ($count <= 10);
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For loops
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The most complicated looping construct is the for loop which has the
following syntax
for (initial-expression;termination-check;loop-end-expression)
statement
When executing a for statement
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The initial-expression is evaluated just once, usually to initialize
variables
The termination-check is evaluated each iteration of the loop and if false,
the loop terminates
The statement or statement block is then executed
The loop-end-expression is then executed and we go back to the
termination-check
for ($count = 0; $count < 10; $count++)
print ($count);
Would output "0123456789"
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Example
<?php
$start_num = 1;
$end_num = 10;
?>
<HTML>
<HEAD>
<TITLE>A division table</TITLE>
</HEAD>
for ($x=$start_num;$x<=$end_num;$x++)
{
print("\n\t<TR><TH>$x</TH>");
for ($y=$start_num;$y<=$end_num;$y++)
{
$result = $x / $y;
// see chapter 10 for printf
printf("<TD>%.3f</TD>",$result);
}
print('</TR>');
}
?>
</TABLE>
</BODY>
</HTML>
<BODY>
<H2>A division table</H2>
<TABLE BORDER="1">
<?php
print("<TR>\n\t<TH>&nbsp</TH>");
for ($x=$start_num;$x<=$end_num;$x++)
print ("<TH>$x</TH>");
print ('</TR>');
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Sample for loop output
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Terminating execution
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Sometimes you just have to give up, and PHP offers a construct that helps
you do just that
The exit construct takes either a string or a number as an argument, prints
out the argument, and immediately terminates the execution of the script
Everything that PHP produces up to the point of invoking the exit
construct is sent to the client browser as usual, and nothing in your script
after that point will even be parsed
Because exit is a construct and not a function, it is also legal to give no
argument and even omit the parenthesis
If you give a number rather than a string to the exit construct, the number
will be the return code of the script
The die construct is an alias for exit
$connection = make_database_connection();
if (!$connection)
die ('Unable to connect to database');
Or
$connection = make_database_connection()
or die ('Unable to connect to database');
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Using functions
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Any real programming language has some kind of capacity for
procedural abstraction - a way to name some pieces of code so
that you can use them as building blocks in writing other pieces
of code
The basic syntax for using (or calling) a function is:
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Function_name(expression_1,expression_2,…,expression_n)
This includes the name of the function followed by a
parenthesized and comma-delimited list of input expressions
(arguments)
Functions can be called with zero or more arguments, depending
on their definition
When PHP encounters a function call, it first evaluates each
argument expression and then uses these values as inputs ot the
function. After the function executes, the returned value (if any) is
the result of the entire function expression
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Return values versus side effects
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Every function call is a PHP expression, and there are only two
reasons why you might want to include one in your code

For the return value


Or the side effects

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This is the value of the function expression itself. You can do exactly the
same things with this value as with the results of evaluating any other
expression (e.g. assign it to a variable or use it in another expression)
Some functions are called for the actions they cause (e.g. writing to files,
manipulating databases, printing things to the browser window, etc.)
Often times functions are called both for their return values and
the side effects at the same time (e.g. calling a function for its
side effect and then testing the return value to ensure that the
function succeeded)
The result of a function may be of any type
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Function documentation
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Although the book covers many of the built-in functions of
PHP, the manual at www.php.net is the authoritative source for
function information (including functions which may be added
for each new release of PHP)
The HTML version of the English PHP documentation is
currently located at http://www.php.net/manual/en/
The online documentation also includes a section for usercontributed notes which may address some particular questions
you may have about the usage of that function, how to combine
it with other functions for a particular effect, or to see what
other side effects that function may cause
Functions are grouped according to their respective categories
with an overview of all of the functions in that category as well
as individual pages for each function built into PHP
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Headers in function documentation
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The very beginning of a function documentation page is
formatted like:

Return-type function-name(type1 arg1, type2 arg2, …)

E.g. string substr(string string, int start[, int length]);



Where the function substr returns a value of type string and takes
2 mandatory arguments of type string and int, and a third optional
argument of type int
Of course PHP will convert almost anything you pass in as an
argument to the type the function is expecting
Return types may also be specified as void or mixed


Return type void is used if the function doesn't return a value
Return type mixed is used if the function may return a value of
any type
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Defining your own functions
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
A function is a way of wrapping up a chunk of code and giving that chunk a
name so that you can use that chunk later in just one line of code
Function definitions have the following form:
function function-name ($argument1, $argument2, …)
{
statement1;
statement2;
return expression;
}
Begin with the key-word function followed by the name you give to that
function followed by your parameter list in parenthesis and the body of the
function inside braces

The name of the function must be made up of letters, numbers, and
underscores and it must not start with a number

Function names are not case-sensitive
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Once called, a function will execute its body until it reaches the end of the
body or until a return statement is reached
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Function definition example
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Suppose you want to create a function which helps decide
which size bottled soft drink to by:
function better_deal ($amount_1, $price_1, $amount_2, $price_2)
{
$per_amount_1 = $price_1 / $amount_1;
$per_amount_2 = $price_2 / $amount_2;
return ($per_amount_1 < $per_amount_2);
}

Which could be used in your code as:
$liters_1 = 1.0;
$price_1 = 1.59;
$liters_2 = 1.5;
$price_2 = 2.09;
if (better_deal($liters_1,$price_1,$liters_2,$price_2))
print ('The first deal is better!<BR>');
else
print ('The second deal is better!<BR>');
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Argument number mismatches
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As can be expected in PHP, if you pass in too few or too many
arguments to a function, it will not crash the application but
may print warning messages depending on your settings

Too few arguments passed:




If you supply fewer arguments to a function than it requires, PHP
will treat the unfilled formal parameters as if they were unbound
variables and may see a warning message printed to the output
You can suppress these warning messages by using the error-control
operator @ which can be placed in front of any expression to suppress
errors from that expression only (e.g. @better_deal(1.0,1.59))
You can also write your function definition to accept a variable
number of arguments by supplying default values (discussed later)
Too many arguments passed:

If you supply more arguments than the function expects, the
extraneous arguments will simply be ignored and no warnings are
issued
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Functions and variable scope
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Each function is basically its own little script & barring some
special declarations, the meaning of a variable name inside a
function has nothing to do with the meaning of that name
elsewhere
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
Exception: the super-global arrays like $_POST, $_GET,
$_SESSION, etc. are automatically visible from within function
definitions
The only variable values that a function has access to are the
formal parameter variables (which have values copied from the
actual parameters), plus any variables that are created inside the
function.

Local variables defined inside a function will have no effect on
code executing outside that function
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Global vs. local
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The scope of a variable defined within a function is local to that function by
default.
This behavior can be overridden by using the global declaration, informing
PHP that you want a variable name to mean the same thing as it does in the
context outside the function
$output_string = 'This is my output';
$separator = '<BR>'."\n";
function modify_string($add_text)
{
global $output_string,$separator;
return $output_string .=
$separator.$add_text;
}
Would output:
This is my output<BR>
Line #2
And the value of the variable
$output_string would be
'This is my output<BR>\nLine #2'
modify_string('Line #2');
print ($output_string);
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Static variables
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By default, functions retain no memory of their own prior execution, and
with each function call local variables act as though they have been newly
created
The static declaration overrides this behavior for specified variables,
causing them to retain their values in between calls to the same function
function count_executions()
{
static $count = 0;
return ++$count;
}
Would output:
12345678910
for ($count=10;$count > 0;$count--)
print (count_executions());
The static keyword allows for an initial assignment, which has an effect
only if the function has not been called before

The second time the function is called, static variables are initially assigned
the value they had at the end of the previous execution
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Recursion

Recursive functions are those functions which call themselves
(either directly or by calling a function which calls itself)
function countdown ($num_arg)
{
if ($num_arg > 0)
{
print("Counting $num_arg");
countdown (--$num_arg);
}
}
Would product the browser output:
Counting 5
Counting 4
Counting 3
Counting 2
Counting 1
countdown (10);

As with all recursive functions, it's important to be sure that the function has
a base case (a nonrecursive branch) in addition to the recursive case, and that
the base case is certain to eventually occur. Otherwise the situation would be
similar to an infinite loop
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Variable numbers of arguments


It's often useful to have the number of actual arguments that
are passed to a function depend on the situation in which it is
called
This can be accomplished by:
1.
2.
3.
Defining the function with default values for the arguments - any
that are missing in the call to the function will have the default
value, and no warning will be printed
Use an array argument to hold the values - it is the responsibility
of the calling code to package up the array, and the function body
must take it apart
Use the variable-argument functions (func_num_args(),
func_get_arg(), and func_get_args()) introduced in
PHP 4
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Default arguments

To define a function with default arguments, simply turn the formal
parameter name into an assignment expression. If the actual call has fewer
parameters than the definition has parameters, PHP will match parameters
with the arguments until the actual arguments are exhausted and then will
use the default assignments to fill in the rest
function open_db ($db_name, $user="MGMT380b", $pwd="MGMT380b",
$host="sql.gettelco.com")
{
$db_handle = mysql_connect( $host, $user, $ pwd)
or die ("Cannot connect to the database server $host:". mysql_error());
mysql_select_db($db_name)
or die ("Error connecting to db $db_name:". mysql_error());
return $dbHandle;
}
open_db ('Store');
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Call-by-value vs. call-by-reference



By default, PHP functions are call-by-value meaning that when
you pass variables to a function call, PHP makes copies of the
values of the variable which are then assigned to the appropriate
parameters used in the function body
Consequently, no matter what a function does, it cannot change
the actual variables that you pass to the function call
If you want the function to actually modify the values of the
variable passed into the function, you can tell PHP to use a callby-reference on the specific parameters in the function
definition by putting an ampersand (&) before the parameter
name (e.g. function my_function(&$modifiable_parameter))
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