PHP - how to get a variable's name?

I want to make a little function for debugging purposes, that prints the contents of the variable in a human readable format.

The function is based on the pr() shortcut found in CakePHP framework, i.e. this one:

I have this so far:

function pr($var,$msg){

    $pr_debug=true;
    if($pr_debug){
        echo "
";
        if($msg) echo "".$msg.": ";
        if(gettype($var)=="array" || gettype($var)=="object" ) echo "
";
        print_r($var);
        echo "
";
    }
}

which prints an optional message ($msg) before the variables content.

But what I want to add, is that if no message is sent, to also print the variable's NAME as such message, so i could get something like this:

$myvar="hello";
pr($myvar); 

//should output:

myvar: hello

So inside my function, how can I get the variable's name as a string so I can output it?
i.e. with pr($foo); I need "foo",
$name="biz"; pr($$name); I need "biz",
and so on...

Preferably I want something that would work despite PHP globals configuration or any of that (which by the way I don't understand very well, so any help on GLOBALS stuff would be much appreciated).

Thanks!

Extra: here at stackoverflow, how do i format source code to get PHP formatting and colors? As of now, I simpy used the toolbar in the textarea and chose "code sample"...