PHP can be used for many things. I have found it to be better for processing forms than Perl CGI scripts. It can be used to construct entire websites. You can make extremely sophisticated web pages with PHP in combination with Java and/or Javascript. PHP also has hooks to connect to many types of database servers (this is probably an incomplete list): MySQL, Microsoft SQL, mSQL, Oracle, PostgreSQL, and ODBC.
PHP is an excellent addition to your webserver, allowing you to write web pages that are responsive to the user's circumstances without having to worry about client-side languages like Java or Javascript, which may or may not be running on their web browser.