From the PHP website: PHP is a server-side, cross-platform, HTML embedded scripting language. PHP is a scripting language like Perl, Python or Tcl. It is the most popular module for Apache and this is due to a variety of reasons:
Learning curve is quite low
Extensive database support
PHP has a modular design. There are modules that provide support for:
Database connetivity for Oracle, ODBC, mySQL, mSQL, PostgreSQL, MS-SQL server... and many more, check the PHP website.
File transfer: FTP
Directory support: LDAP
Mail support: IMAP, POP3, NNTP
PDF document generation
and many more. You only need to compile/use the modules you need.
PHP can be used with Apache, as an external CGI or with other webservers. It is crossplatform and it runs on most flavors of Unix and Windows.
If you come from a Windows background, you probably have used Internet Information Server with Active Server Pages and MS-SQL Server. A common replacement in the Unix world for this trio is Apache with PHP and mySQL. Since PHP works: