I don't know what a tooltip is I'm assuming it is like when you have your mouse over a link it provides a little blurb. Just a reminder, php is server side and does not deal with client side functions. You are possbily looking for html or javascript.
Most likely you want to use the TITLE attribute. Possibly you want to use the ALT attribute for image tags, which is improperly implemented in IE to behave as a TITLE attribute. Even though it's wrong, it's the de facto standard.