If you're using one's compliment (bitwise NOT) to flip all $a's bits, don't forget that you need to use a mask to specify how many bits you expect in your result:
<?php gmp_and(gmp_com('1234'), '2047') ?>
>> 813 // 01100101101 in binary
I chose 2047 because 1234 is an 11-bit binary number and 2047 is the maximum an 11-bit register can hold:
11111111111 (2047)
10011010010 (1234)
01100101101 (813)