SQL

 A DBM database is a good option when you want to store small amounts of relatively simple data (typically name/value pairs). Scripts built to use a DBM database have the virtue of portability. If you intend to store large amounts of data or many fields, consider using a SQL database, such as MySQL.
SQL stands for Structured Query Language. It provides a standardized syntax by which different types of database can be queried. Most SQL database products provide their own extensions to the language, just as many browsers provide their own extensions to HTML. Nonetheless, an understanding of SQL enables you to work with a wide range of database products across multiple platforms.
PHP is designed to integrate easily with databases. This feature is one of the factors that make the language such a good choice for building sophisticated Web applications. Many databases are directly supported, including Adabas D, InterBase,Solid, dBASE, mSQL, Sybase, Empress, MySQL, Velocis, FilePro, Oracle, UNIX dbm, Inform ix, and PostgreSQL. PHP also supports ODBC.