PHP, OOP and AJAX

Hi Everyone, This is the first time that I am posting in the PHP forum, so hope that I am osting in the right place. I would like to say that before submitting to this forum I have done some research looking for a solution without success. I had been programming in ASP.NET for years using Object Oriented Princeliness but decided to walk away from that. I am now researching and specialising in the open source world. I have started to develop a project using MySQL, PHP and OOP. So far I have succeed. However I got stuck once I started implement AJAX using the AJAX tutorial from w3schools.com. What I have discovered is: for some reason when you call a file that requires other fies using the REQUIRE or INCLUDE it just does not work. I can conform this as I have tested with out the the functions. Has anyone ever meet such a situation can you give me some feedback please. Thank you very much in advance for your support. Regards Julian