Installing/Configuring
PHP Manual

Requirements

The behaviour of various aspects depends on the implementation of the underlying database. Functions such as dba_optimize and dba_sync will do what they promise for one database and will do nothing for others. You have to download and install supported dba-Handlers.

List of DBA handlers
Handler Notes
dbm Dbm is the oldest (original) type of Berkeley DB style databases. You should avoid it, if possible. We do not support the compatibility functions built into DB2 and gdbm, because they are only compatible on the source code level, but cannot handle the original dbm format.
ndbm Ndbm is a newer type and more flexible than dbm. It still has most of the arbitrary limits of dbm (therefore it is deprecated).
gdbm Gdbm is the » GNU database manager.
db2 DB2 is for » Oracle Berkeley DB 2. It is described as "a programmatic toolkit that provides high-performance built-in database support for both standalone and client/server applications.
db3 DB3 is for » Oracle Berkeley DB 3.
db4 DB4 is for » Oracle Berkeley DB 4 or 5. This option is available from PHP 4.3.2. It can be used with BDB 5 from PHP 5.3.3.
cdb Cdb is "a fast, reliable, lightweight package for creating and reading constant databases." It is from the author of qmail and can be found at » http://cr.yp.to/cdb.html. Since it is constant, we support only reading operations. And since PHP 4.3.0 we support writing (not updating) through the internal cdb library.
cdb_make Since PHP 4.3.0 we support creation (not updating) of cdb files when the bundled cdb library is used.
flatfile This is available since PHP 4.3.0 for compatibility with the deprecated dbm extension only and should be avoided. However you may use this where files were created in this format. That happens when configure could not find any external library.
inifile This is available since PHP 4.3.3 to be able to modify php.ini files from within PHP scripts. When working with ini files you can pass arrays of the form array(0=>group,1=>value_name) or strings of the form "[group]value_name" where group is optional. As the functions dba_firstkey and dba_nextkey return string representations of the key there is a new function dba_key_split available since PHP 5 which allows to convert the string keys into array keys without loosing FALSE.
qdbm This is available since PHP 5.0.0. The qdbm library can be loaded from » http://fallabs.com/qdbm/index.html.

When invoking the dba_open or dba_popen functions, one of the handler names must be supplied as an argument. The actually available list of handlers is displayed by invoking phpinfo or dba_handlers.