You could use two 32-bit numbers, four 16-bit numbers, etc.
PHP has rand() and and mt_rand() but how many random bits they supply isn't specified by the standard (though they can be queried with the help of getrandmax() and mt_getrandmax(), respectively.)
safest simplest bet would be generating 64 random bits and setting them one by one.
As for working with 64-bit integers, I'd recommend using the GMP library as it has a good range of functions to help you out.
You could create a number, call 64 gmp_setbit()s on it with successive positions then convert it to a string using gmp_strval().