If you want to convert a decimal (integer) number into constant length binary number in lets say 9 bits, use this:

$binary = sprintf('%08b', $number );

for example: 
<?php
$bin = sprintf('%08b',511 );
echo $bin."\n";
?>

would output 111111111
And 2 would output 00000010

I know the leading zeros are useful to me, perhaps they are to someone else too.