Stands for: PHP: Hypertext Preprocessor
PHP is a scripting language mainly designed for web development, where it is quite popular.
This [L1 ] is PHP's home page.
For Tcl's answer to PHP, see mod_dtcl or Rivet.
In the last few years, "PHP [has become] Handy off the Web, Too", although this article [L2 ] doesn't recommend it for those who already know Tcl and similar languages.