This has been disuceesd in The Manual's user comments. Here's a post on that
topic by "bishop" quoted from
http://fi.php.net/manual/en/functions.arguments.php (wouldn't hurt to take a
look at the entire page)

"Functions explicitly prototyped with formal parameters passed by reference
can't have default values. However, functions prototyped to assign default
values to formal parameters may be passed references.

For example, this is a parse error:

function foo(&$bar = null) {
// formal parameters as references can't have default values
$bar = 242;
}

While this is perfectly legal (and probably what you want, mostly):

function foo($bar = null) {
$bar = 242;
}

foo(); // valid call, no warnings about missing args
foo(&$x); // valid call, post $x == 242
"

And this has been asked in here also earlier...

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PHP doesn't pass arrays or objects by reference for default (well, at least
not until PHP 5, if I remember well). If you want to use an optional
argument, you can use the func_get_arg() passing the argument number you
want (0 for the first). In that case the function would get:

function funQueryDatabase($strQuery /*, &$intInsertId*/) {
/*...*/
if ((func_num_args() > 1) {
$id =& func_get_arg(1);
/*...*/
}
/*...*/
}

And you could call it with funQueryDatabase($str, 10) or
funQueryDatabase($str, &$id).
In this way you avoid the need to pass a variable name for the second
parameter (required if the formal parameter is in the form &$id, which would
not permit to use a number or a string).


I hope this answers to your question.
--
Xenophaw