I'm teaching myself PHP right now, so maybe I can answer.
PHP-the-langauage gets parsed and executed pretty rapidly so you won't notice a huge performance drop when generating HTML, rather than just having a bunch of static HTML files.
PHP-the-interpreter has library calls for just about every open source DBMS, and a lot of the proprietary ones, too, so making a 3-tier-style web app looks easy. PHP-the-interpreter includes library calls for a lot of other commons situations (a.k.a. "use cases"), so you don't have to write a lot of code yourself, just call the library. So, just like J2EE "enterprise apps", the problem moves from knowing how to code to knowing how to look library stuff up. This is aided by the very decent php.net PHP manual.
As a programming language, PHP doesn't have a lot of exotic constructs, or differ too much from, say C, FORTRAN 4 or Pascal, so programmers that already know one language can easily pick up PHP-the-programming language.