Just to add to some of the other good answers you have received here, PHP is a server side scripting language much like ASP.
I like PHP because it is fast and "code" centric. Programmers from other programming languages (like C, Perl, Delphi, VB, etc.) find it very comfortable. This is in contrast to a "html" centric tool like Cold Fusion, where all commands are in tags.
Secondly, some reviewers and "experts" claim that PHP's lack of a single database abstraction layer is a weakness. Maybe in some circumstances, but normally, I'd rather have the use of database functions optimized for my target database. PHP has this in spades. (BTW, PHP does have an abstraction layer, PEAR, included in the 4.0 distributions. It is just in beta and not "finished" yet.)