PHP Manual
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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.';
}
$str = 'This is a string, ';
add_some_extra($str);
echo $str;    // outputs 'This is a string, and something extra.'

If you wish to pass a variable by reference to a function which does not do this by default, you may prepend an ampersand to the argument name in the function call: 

function foo ($bar) {
    $bar .= ' and something extra.';
}
$str = 'This is a string, ';
foo ($str);
echo $str;    // outputs 'This is a string, '
foo (&$str);
echo $str;    // outputs 'This is a string, and something extra.'