Ten years on, Brian's note is still a good overview of how offsets work, but a shorter and simpler summary is:
strrpos($x, $y, 50); // 1: this tells strrpos() when to STOP, counting from the START of $x
strrpos($x, $y, -50); // 2: this tells strrpos() when to START, counting from the END of $x
Or to put it another way, a positive number lets you search the rightmost section of the string, while a negative number lets you search the leftmost section of the string.
Both these variations are useful, but picking the wrong one can cause some highly confusing results!
