PHP5 configure options

I'm building PHP 5RC1 and 4.3.6 on a RH ES3 Linux box. 4.3.6 builds and runs perfectly - the same configure options on PHP 5 produces a build, but gives runtime errors about undefined symbols at PHP startup (e.g. In response to php -v). I then failed to find any docs on differences in compile time options between 4 and 5 - I assume there must be some! Using ./configure with no options builds and runs OK (but is obviously missing stuff), so it must be something that's changed on the options that's to blame. http://www.php.net/manual/en/install.configure.php does not mention PHP5 at all - is there some other site where php5 documentation lives? Marcus