MySQL, PostGreSQL, SYBASE, IBM-DB2, Oracle Database and Mongo are widely used databases.
Below are some other databases supported by php
•    Cubrid
•    DB++
•    dBase
•    filePro
•    FireBird/InterBase
•    FrontBase
•    Informix
•    Ingres
•    MaxDB
•    mSQL
•    Ovrimos SQL
•    Paradox
•    SQLite
•    SQLite3
•    SQLSRV
•    Tokyo Tyrant