Actually, shtml is much more powerful than most folks realize. It has conditionals, variable substitution, regular expression comparisons, and the ability to echo server variables and formatted strings. There's a lot more you can do with those tools than simply including files. A company where I used to work ran a network of 30 or so locally branded sites with extensive use of shared content automatically mixed with local content, and made it all happen with nothing more sophisticated than XSSI directives.
PHP does everything XSSI offers, plus math, much more powerful string manipulation, the ability to define and use functions and objects, network transparency (fetch a file from a remote url), and access to external databases such as MySQL. The language itself is fairly small, simple, and clear, especially to anyone who's ever programmed in a C-like language. Most of the power comes from its ability to include third-party function libraries such as the ones that support the various databases.
