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OK, I had the same problem, for a slightly different reason.
RedHat 9.0, freshly up2date, php-4.2.2.. but I then installed MySQL 4.0.18 (NOT 3.23, which is what XOOPS says it needs on its install wizard/druid pages).
OK, this shouldnt have been a problem, but php-mysql wasnt installed... weird, ok, I had NO MySQL functions.. but wait, it seems to want the MySQL RPM (according to synaptic anyway), which is from MySQL 3!
Short solution, install MySQL-shared-compat-4.0.18-0.i386.rpm, which includes the specific lib file that 3.23 provided, and PHP still links to. Now I have finally seen the light, well, the next page of the installation anyway, and I'm eagerly awaiting to find out if this was worth the trouble. But then, the learning was worth the price of admission, eh?