There should be a function which returns current functions name.

EXAMPLE:

function foo(){
    debug("Entering function ". func_get_name() ."<br>");
    // Function definition here.
}

function debug($msg){
    echo "<span class=debughdr>Debug information: </span>";
    echo "<span class=debugmsg>$msg</span>";
}

I think func_get_name would be an appropriate name. At least that was the name I looked for it.

I don't know any workaround nor how to make user defined function providing this functionality but after asking Experts Exchange I've convinced that I'm not the only one requiring this.