Pass the logged in username the sAjaxSource

Guys,

I'm having trouble passing the logged in username (WordPress) to the sAjaxSource php file so I can mysql_query appropriately. I've tried to globalize my username variable, use an include.php file with the username in it to no avail..

How do I pass a php variable into the sAjaxSource file from my php file that displays the data??

Thanks!