//define code to override mail function (note I've used PHP5.3 Nowdoc Syntax to avoid 
//the need to escaPE the dollar symbols!!
$code=<<<'CODE'
    $trace=debug_backtrace();
    $caller=array_shift($trace);

    echo 'mail() called by '.$caller['function']
    if (isset($caller['class']))
        echo 'in '.$caller['class'];
CODE;

//install override
override_function('mail','$to,$subject,$msg,$hdrs,$params',$code);