[RESOLVED]What's so special about this composite number

15999

I had written a program in PHP that determined whether an integer is composite or prime. Under certain versions of EasyPHP, 15999 would cause the virtual server to crash in a very abnormal way. I tested numbers close to 15999 and numbers much larger and I was unable to repeat the problem with any other number. From a mathematical perspective is there anything unique about this composite number?

When I did a google search it came up with pages about pseudo primes, but my program was deterministic so that doesn't make sense.