Have you got the seating plan worked out. How are you doing it.
Two ways, a simple table (for a simple venue) or an image for a complex venue (say arced like the circle at the theatre). If image then use image map, if table then simple clicks.
Either way, you map it so that the hyperlink is <a href="bookit.php?seat=123&showid=123">Seat 123</a>
I'd use a template to parse in the showid which the user is looking at. If you used phplib (free) the html would look like <a href="bookit.php?seat={SEAT}&showid={SHOWID}">Seat {SEAT}</a>
Then in your php you'd run through the available seats and parse in the seat number and the showid. The only minor problem with this approach is a bug in phplib but leave your email and I can send you a good version.
Better still, you know, is to parse in the whole thing from <a to </a> and then if the seat is booked you'd just give the seat number without the link so people can't click on it.
Should be fun to do, and shouldn't be too hard.
Good luck