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][a-zA-Z0-9_\x7f-\xff]*.
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 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.
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. 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 #4 Recursive functions
<?php
function recursion($a)
{
if ($a < 20) {
echo "$a\n";
recursion($a + 1);
}
}
?>
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