ok, all you need is your cron to execute the script everyday.
and the script would be something like that :
$date=date("YmdHis", mktime(0,0,0,date("m"),date("d")-15,date("Y"))); // this compute the date of today -15 days.
$query="select * from users where paid='no' and date_inscription='".$date."'";
$result=mysql_query($query);
while($row=mysql_fetch_array($result)) {
mail( ... all the info needed in here ... )
}
this script has just one flaw, it will send the mail only once (when 15 days have passed, never after)
you could change it by sending one mail each day after the 15,
then the query would slightly change to this :
$query="select * from users where paid='no' and date_inscription<='".$date."';
