Not for 'how-to' coding questions but PHP theory instead, this forum is here for those of us who wish to learn about design aspects of programming with PHP.
It's a big integer - way past the 32bit limit. So it's a float, not an integer. Like AC said, you need BCMath, or preferably GMP (it's a great deal faster than BCMath).
Yeah it's the largest unsigned 64 bit integer I believe. I ended up using the gmp family of functions. Apparently that's only (64 bit) number that I couldn't accurately reproduce on a 64 bit processor.