gmp_testbit will be very useful for my (over 64-bit) bitmask checking.
In the meantime, I think these are the best replacements.
Mostly full:
<?php
(gmp_scan1($a, $index) == $index)
?>

Mostly empty:
<?php
(gmp_scan0($a, $index) != $index)
?>

Still worst-case O(N), though.