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most should be ok with php 5 (altho a few modules may cause issues, but not many especially the ones that are actively being developed now in the last year)
i haven't tested using MySQL 5 as i don't feel the need to upgrade to 5 yet on my testing pc. php5 is essential when using mysql5, altho php4 will work with mysql5 it can't make use of all the new features that mysql5 contains.
unfortunately you'd be better to ask some1 experienced on how to downgrade your php/mysql on suse, as i'm not sure myself, my friend did mine as i wanted php4.4 and MySQL 4.1 as that's what my websites servers are using.. i'spose u could just uninstall them and install the 4.x version RPM's.. i'm a linux newbie tho, windows is my main O/S
i did find this article tho which may help >
http://www.aditus.nu/jpgraph/apache2suse10.php