PHP allows, among other things, to easily interact with a very large number of RDBMS (Oracle, DB2, MySQL, PostgreSQL, ...), while maintaining a very simple and straightforward syntax.
Its ease of use and similarity with the most common structural programming languages, allowed most experienced programmers to start developing complex applications with a minimal learning curve.
PHP was in September of 2001 installed in over 900.000 internet servers, proving how easily it has been adopted by the open-source community.
Today PHP has grown through the collective efforts of many contributors. It is on the Public Domain, and covered by the GPL.
You can find information about PHP on the home page at http://www.php.net