When you tell us what you want to make, maybe we can help you to solve the problem or we give you some ideas.
I mean: why you need a client-side event for PHP?
Thoaren, "serverside" means that calling a PHP function, or script, would always require a reload of the page. AJAX is a technique to load a file (just like reloading), but then within the current page. However, in most cases there's a better workaround than AJAX. So in interest of performence, and in interest of good code, please tell us your exact problem.
I have a header.php with menu in it.
When clicked on one of the menu-items, header.php and the corresponding page from the menu is been included in index.php.
This a will not change.
So this is the situation description:
*index.php:
-includes header.php
-includes corresponding page from de menu in header.php
-includes footer.php
*header.php with menu
*corresponding pages, let's say: page01.php and page02.php
This I will not change.
Now I want to change the backgroundimage of a cell in the menu when the corresponding page, let's say page02.php is loaded in the index.php. The backgroundimage of the other menu-items must be change to standard then. I want to do the same when page01.php is loaded in index.php. Also the backgroundimage of the other menu-items must be change to standard then.