(PHP 5, PHP 7 >= 5.3.0)
gmp_testbit — Tests if a bit is set
   Returns TRUE if the bit is set in resource $a, 
   otherwise FALSE.
  
   An E_WARNING level error is issued when 
   index is less than zero, and FALSE
   is returned.
  
Example #1 gmp_testbit() example
<?php
$n = gmp_init("1000000");
var_dump(gmp_testbit($n, 1));
gmp_setbit($n, 1);
var_dump(gmp_testbit($n, 1));
?>
The above example will output:
bool(false) bool(true)