PHP is basically a very low-level entry into programming, it's easy to pick up, and as mentioned already it's free.
There's also an ease of installation; most PHP scripts these days are capable of more or less auto-installing themselves. Java apps require a lot of kicking the server around to make it do what you want, and unfortunately most Perl scripts also suffer from the same problem in the sense that it often requires command line access at the very least and root access usually to get all the additional modules installed.
Personally I write all my stuff in Perl since that's what I've been using for the past 15 years, and personally hate PHP's guts for it being such a security hole ridden piece of shit
