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# Chapter 17. Functions

# User-defined functions

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

Example 17-1. 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.

Function names follow the same rules as other labels in PHP. A valid function name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*.

In PHP 3, functions must be defined before they are referenced. No such requirement exists since 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 17-2. 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 17-3. 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(); ?>`

All functions and classes in PHP have the global scope - they can be called outside a function even if they were defined inside and vice versa.

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). Both are supported, as of PHP 4: see Variable-length argument lists and the function references for func_num_args(), func_get_arg(), and func_get_args() for more information.

It is possible to call recursive functions in PHP. However avoid recursive function/method calls with over 100-200 recursion levels as it can smash the stack and cause a termination of the current script.

Example 17-4. Recursive functions
`<?php function recursion($a) { if ($a < 20) { echo "$a\n"; recursion($a + 1); } } ?>`

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