User-defined functions
A function may be defined using syntax such as the following:
Example #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*.
Tip
See also the Userland Naming Guide.
Functions need not be defined before they are referenced, except when a function is conditionally defined 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 #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 #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?processing?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.
Both variable number of arguments and default arguments are supported in functions. See also 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.
Example #4 Recursive functions
<?php
function?recursion($a)
{
????if?($a?<?20)?{
????????echo?"$a\n";
????????recursion($a?+?1);
????}
}
?>Note: Recursive function/method calls with over 100-200 recursion levels can smash the stack and cause a termination of the current script. Especially, infinite recursion is considered a programming error.
Function arguments →
? 1997–2017 The PHP Documentation GroupLicensed under the Creative Commons Attribution License v3.0 or later.
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