Seeing as people keep asking why I think that PHP Fails It, here are some reasons (I think it's fine for "SSI with extra bits", but not as a serious web application development language, even for relatively trivial applications like online discussion forums):

(Firstly, this list was drawn up while PHP 5.0 was very new and very buggy, and thus largely unavailible for any real business application (i.e. now).)

• Horribly inconsistent API. Half the functions are of the form array_thingy, half arraythingy, and some plain thingy thrown in for good measure. (While Java's API has cruft in it, it's not this bad. Perl has no such sillyness, with a small core of functions and proper, modular use of packages (each with its own namespace) to add to it.)
• Appalling MySQL integration. No placeholder binding! You're just BEGGING for someone to inject malicious SQL. There's no decent database abstraction layer like JDBC or Perl DBI.
• Concerning implementation. There are crazy limitations that stink of a poorly implemented underlayer. For example, the way you can't do foo->bar()->baz(), where foo->bar() returns something you can do baz() to. (Neither Perl nor JSP are thusly broken.)
• Virtually no type-checking. While scripting languages are not known for good type-checking, any serious project requires actual programming not hacking. PHP forces you to do the latter. (Perl allows some type-checking; JSP's is much stronger.)
• No exceptions. This is error handling dating back to the BASIC days. (Meanwhile, JSP and Perl allow throwing exception objects.) Even in PHP5, they aren't thrown by most standard functions.
• You can't pass NULL as an argument when the function expects a reference.

All in all, PHP is a language designed and implemeted by and for script kiddies. OK for a quick hack (heck, my website uses it), but no good for real web application development.