Yes - server side validation is mandatory, client side is nice to have.
If you are talking about the form returned to the user, all that PHP can do is send back a stream of html/js which can contain your .focus() instruction so that the required form element lights up or whatever.
You are best off adding a html message as well in case JS is turned off/unavailable.
echo "<p>Please add your name</p>";
echo "<input type=text name=fname id=fname '/>";
That is a simple example of a way of doing it for illustrative purposes, I'm sure you can improve upon it and integrate it.