Python has ctypes to access libraries. In PHP you write extensions for everything. Popular extensions like the one for libgd are available almost everywhere.
I don't think there is such a thing : in PHP, the "standard" way of using a library is by writting a wrapper arround it, that exports the functions of the library to PHP.
(But maybe an extension could be written to do just what ctypes does ? -- Not sure, but maybe ^^ )