You can work out the number of seconds between two timestamps just by subtraction. I'd suggest getting a timestamp of the birthday with the function [MAN]strtotime[/MAN], and the timestamp of the current date with [MAN]mktime()[/MAN]. You'll probably want to use next year's birthday if one of the current year has already passed. This can be done with strtotime too, as follows
$date = '2006-04-08';
$next_year = strtotime( $date . ' +1 year' );
Have a go at making a function to do this bit of processing.
