Ajax call php function

Hello I want a script working with ajax php and mysql It's need to use XML HTTP REQUEST in often time is what i read in tutorials i want my web page use ajax to call a php function Security (the visitor can't choose the function to call by ajax) Example of use i have a div witch contain number of connected Ajax call a function who count number of connected (i've already make the function count) I want to recuperate all php var in array and assign it to smarty for display my webpage After ajax refresh the div i need parameter refreshing rate in seconds Ajax can call function to get information in my mysql base and refresh a specified div The information refreshed displayed have to be in HTML NO WESTERN UNION PAYMENT NO BID FOR WESTERN UNION PAYMENT Don't hesitate to contact me