User-defined functions
Returning values
Functions
PHP Manual

## Function arguments

Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. The arguments are evaluated from left to right.

PHP supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists are also supported, see also the function references for func_num_args, func_get_arg, and func_get_args for more information.

Example #1 Passing arrays to functions

`<?php
function takes_array($input)
{
echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
?>`

### Making arguments be passed by reference

By default, function arguments are passed by value (so that if the value of the argument within the function is changed, it does not get changed outside of the function). To allow a function to modify its arguments, they must be passed by reference.

To have an argument to a function always passed by reference, prepend an ampersand (&) to the argument name in the function definition:

Example #2 Passing function parameters by reference

`<?php
function add_some_extra(&$string)
{
$string .= 'and something extra.';
}
$str = 'This is a string, ';
add_some_extra($str);
echo $str; // outputs 'This is a string, and something extra.'
?>`

### Default argument values

A function may define C++-style default values for scalar arguments as follows:

Example #3 Use of default parameters in functions

`<?php
function makecoffee($type = "cappuccino")
{
return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>`

The above example will output:

    Making a cup of cappuccino.
    Making a cup of .
    Making a cup of espresso.

PHP also allows the use of arrays and the special type `NULL` as default values, for example:

Example #4 Using non-scalar types as default values

`<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
$device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>`

The default value must be a constant expression, not (for example) a variable, a class member or a function call.

Note that when using default arguments, any defaults should be on the right side of any non-default arguments; otherwise, things will not work as expected. Consider the following code snippet:

Example #5 Incorrect usage of default function arguments

`<?php
function makeyogurt($type = "acidophilus", $flavour)
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // won't work as expected
?>`

The above example will output:

    Warning: Missing argument 2 in call to makeyogurt() in
    /usr/local/etc/httpd/htdocs/phptest/functest.html on line 41
    Making a bowl of raspberry .

Now, compare the above with this:

Example #6 Correct usage of default function arguments

`<?php
function makeyogurt($flavour, $type = "acidophilus")
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // works as expected
?>`

The above example will output:

    Making a bowl of acidophilus raspberry.

> Note: As of PHP 5, arguments that are passed by reference may have a default value.

### Variable-length argument lists

PHP has support for variable-length argument lists in user-defined functions. This is really quite easy, using the func_num_args, func_get_arg, and func_get_args functions.

No special syntax is required, and argument lists may still be explicitly provided with function definitions and will behave as normal.