Perl vs PHP: Last I checked, DBI was just a better "low level" database interface than anything PHP has to offer, but that might have changed in the last 2 - 3 years. But most of the differences are in the ease of using safe and efficient constructs and unicode support (which is pretty good in DBD::mysql)

In any case, learning DBI or whatever PHP database interface is fairly trivial once you've got your SQL/MySQL skills up to scratch. The interfacing language isn't really that important, as far as database programming is concerned.