This sort of thing is an intrinsic property of floating point math. It has limited precision. When the numbers get sufficiently large, that precision is insufficient to distinguish successive integers. That is to say, this is symptomatic of PHP implementing floating point stuff correctly.

This is what ghci (Haskell) says:

  Prelude> 9223372036844775807 == 9223372036844775808
  False
  Prelude> 9223372036844775807.0 == 9223372036844775808.0
  True

This is what Python says:

  >>> 9223372036844775807 == 9223372036844775808
  False
  >>> 9223372036844775807.0 == 9223372036844775808.0
  True

Here is what SBCL (Common Lisp) says:

  * (= 9223372036844775807 9223372036844775808)
  NIL
  * (= 9223372036844775807.0 9223372036844775808.0)
  T

Lua:

  > print(9223372036844775807 == 9223372036844775808)
  true (!!!!!)
  > print(9223372036844775807.0 == 9223372036844775808.0)
  true

Javascript:

  alert(9223372036844775807 == 9223372036844775808)
  true (!!!!!)
  alert(9223372036844775807.0 == 9223372036844775808.0)
  true

Other languages that will also do this[1]: Javascript, Lua. Languages that won't: anything with actual, honest-to-god integers, and not floats or doubles masquerading as them.[2] Languages that actually handle numbers sensibly: Lisp.[3] I'm not familiar with any others that actually treat rational numbers like rational numbers, but I expect there are some. (It's still, of course, impossible to treat real numbers like real numbers, meaning that this sort of thing will also happen there.)

[1] Well, not the string-to-number bit, but whatever.

[2] Except for the niggle that they'll still do this when you're using floating point numbers, because this is what floating point numbers do.

[3] https://en.wikipedia.org/wiki/Numerical_tower