<?php $number=55; echo str_pad($number, 6, '0', STR_PAD_LEFT); ?>
above will print 000055
<?php $number=55; sprintf('%06d', $number); ?>
above will also print 000055
<?php $number=55; echo str_pad($number, 6, '0', STR_PAD_LEFT); ?>
above will print 000055
<?php $number=55; sprintf('%06d', $number); ?>
above will also print 000055