Function arguments

Information may be passed to functions via the argument list, which is a comma-delimited list of variables and/or constants.

PHP supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists are supported only in PHP 4 and later; see Variable-length argument lists and the function references for func_num_args(), func_get_arg(), and func_get_args() for more information. A similar effect can be achieved in PHP 3 by passing an array of arguments to a function:

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 you change the value of the argument within the function, it does not get changed outside of the function). If you wish to allow a function to modify its arguments, you must pass them by reference.

If you want an argument to a function to always be passed by reference, you can prepend an ampersand (&) to the argument name in the function definition:

function add_some_extra(&$string)
    $string .= 'and something extra.';
$str = 'This is a string, ';
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:

function makecoffee ($type = "cappuccino")
    return "Making a cup of $type.\n";
echo makecoffee ();
echo makecoffee ("espresso");

The output from the above snippet is:
Making a cup of cappuccino.
Making a cup of espresso.

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

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:

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

The output of the above example is:
Warning: Missing argument 2 in call to makeyogurt() in 
/usr/local/etc/httpd/htdocs/php3test/functest.html on line 41
Making a bowl of raspberry .

Now, compare the above with this:

function makeyogurt ($flavour, $type = "acidophilus")
    return "Making a bowl of $type $flavour.\n";
echo makeyogurt ("raspberry");   // works as expected

The output of this example is:
Making a bowl of acidophilus raspberry.

Variable-length argument lists

PHP 4 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.