Chapter 12. Functions
User-defined functions
A function may be defined using syntax such as the following:
Example 12-1. Pseudo code to demonstrate function uses
<?php
function foo ($arg_1, $arg_2, /* ..., */ $arg_n)
{
echo "Example function.\n";
return $retval;
}
?>
Any valid PHP code may appear inside a function, even other functions and class definitions.
In PHP 3, functions must be defined before they are referenced. No such requirement exists in PHP 4. Except when a function is conditionally defined such as shown in the two examples below.
When a function is defined in a conditional manner such as the two examples shown. Its definition must be processed prior to being called.
Example 12-2. Conditional functions
<?php
$makefoo = true;
/* We can't call foo() from here
since it doesn't exist yet,
but we can call bar() */
bar();
if ($makefoo) {
function foo ()
{
echo "I don't exist until program execution reaches me.\n";
}
}
/* Now we can safely call foo()
since $makefoo evaluated to true */
if ($makefoo) foo();
function bar()
{
echo "I exist immediately upon program start.\n";
}
?>
Example 12-3. Functions within functions
<?php
function foo()
{
function bar()
{
echo "I don't exist until foo() is called.\n";
}
}
/* We can't call bar() yet
since it doesn't exist. */
foo();
/* Now we can call bar(),
foo()'s processesing has
made it accessible. */
bar();
?>
PHP does not support function overloading, nor is it possible to undefine or redefine previously-declared functions.
Note: Function names are case-insensitive, though it is usually good form to call functions as they appear in their declaration.
PHP 3 does not support variable numbers of arguments to functions, although default arguments are supported (see Default argument values for more information). PHP 4 supports both: see Variable-length argument lists and the function references for func_num_args(), func_get_arg(), and func_get_args() for more information.
