User-defined functions

A function may be defined using syntax such as the following:

Example 12-1. Psuedo 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.

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.