PHPFusion is compatible with PHP 4, PHP 5, MySQL 4.x, MySQL 5.x and newer. You don't have to worry. However, if you don't know how to backup your site and database, you can leave this to your web host. As you can see, most of the people are already using PHP 5 and I'm pretty much sure a large part of them are using MySQL 5.x as well. Just make sure you update PHPFusion to v7.00.07 and everything will be fine.
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