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What about MySQL less than 5.0.15?

MySQL has discontinued active development for all version below 5.0. The Phorum teams uses 5.0 for all development. MySQL 5.0.15 was the first production release of the 5.0 branch. Phorum has been known to work with MySQL 4.1 and some later 4.0 versions. However, there is no testing with these version. It is recommended that all users upgrade to 5.0 as soon as possible to get the most out of MySQL and Phorum. And yes, I know Google uses MySQL 4. But, remember, they also patch MySQL themselves. If you can patch MySQL yourself, then by all means, go for it.