If you don't have gmp extension enabled (or don't want to use it for any reason), you can get popcount of an int using decbin() and substr_count().
<?php
$int1 = 133;
$bin1 = decbin($int1); // "10000101"
echo substr_count($bin1, "1");
// Result: 3
?>
Being a string-comparison this is far less efficient than gmp_popcount() (for which there is a dedicated instruction on most if not all modern processors), but may be handy if gmp is unavailable, or in non-performance-critical code that doesn't otherwise need it.