The MOODLE_18_STABLE branch has an outdated /search/Zend directory. The updates applied to /search/Zend in MOODLE_19_STABLE should be back ported to MOODLE_18_STABLE.
I believe that I am experiencing problems with PHP 5.2.5 with the Zend API and on a Windows OS Box, the set_include_path doesn't seem to work in /search/tests/index.php. The Zend API in MOODLE_19_STABLE seems to fix both problems.