You CAN use mysql or postgresql, but in this case gdbm is used. Don't ask me why...
The following is a snip from the index.php file:
$DBParams = array(
// Select the database type:
//
// Choose ADODB or SQL to use an SQL database with ADODB or PEAR
// respectively (both ADODB and PEAR libraries are already included
// with PhpWiki).
// Choose dba to use one of the standard UNIX dbm libraries.
// Choose file to use a flat file database.
//'dbtype' => 'ADODB',
//'dbtype' => 'SQL',
'dbtype' => 'dba',
//'dbtype' => 'file',
//'dbtype' => 'cvs',
// Used by either 'dba' or 'file' and must be writable by the web
// server If you leave this as '/tmp' you will probably lose all
// your files eventually
'directory' => "/home/e-smith/files/ibays/phpwiki/html/pages",
// choose the type of DB database file to use; most GNU systems have gdbm
'dba_handler' => 'gdbm', // Either of 'gdbm' or 'db2' work great for me.
//'dba_handler' => 'db2',
//'dba_handler' => 'db3', // Works fine on Windows, but not on every linux.
//'dba_handler' => 'dbm', // On sf.net redhat there's dbm and gdbm.
// dbm suffers from limits on size of data items?