Stop PHP execution on client connection closed

Hi all, I'm new in the list and I already have a question for you. I'm running an heavy query on my DB in a PHP script called by AJAX. Because client often abort AJAX connection to ask a new query, I need to stop query because DB will be too loaded. When AJAX connection is aborted, PHP script doesn't stop until it send some output to client, so I need to wait query execution to know client aborted connection. How can I abort query (or script) when AJAX connection is aborted? Thank you, Marco