The if() statement tests for a true(1)/false(0) condition. Any other value than true or false would make no sense to the interpreter. Therefore $seatno is either true or false, or else it doesn't work.
To make your if() statement work, you'd need to have the statement evaluate a test scenario like the examples below:
if($seatno=="") == If $seatno isn't set to any value (is NULL)
if($seatno<>"") == if $seatno isn't NULL
if($seatno > "0") == if $seatno is greater than 0
Any of these statements would either evaluate to true or false. If the statement evaluates as true, PHP will process the if() loop. If not, it will pass it over.
Does this make more sense?