Try this:
$seats = 7;
for ($i=1; $i <= $seats; $i++) {
?>
<div class='col-xs-6'>
<div class='well text-center <?php echo (in_array($i, $booked_seat)) ? "booked" : "" ?>' id='<?php echo $i ?>'>
Seat No: <?php echo $i ?>
</div>
</div>
<?php
}
?>
warning: code is untested.