PHP uses the dotted integer convention for versions (i.e. the version number should be considered a sequence of integers, not a floating point number). 5.3.10 was the release after 5.3.9. That means 5.3.18 is sixteen releases after 5.3.2.
Note that this is the standard way of interpreting a version number with more than one dot in it. Single-dotted version numbers are less consistent. Some projects consider them a floating point number, and some treat them as a pair of integers.