The Relational DAS requires that the SDO extension be installed. The SDO extension requires a version of PHP 5.1, and the Relational DAS requires a recent version that contains an important fix for PDO. The most up-to-date information about required levels of PHP should be found in the changelog for the package on PECL. At the time of writing, though, the Relational DAS requires the most recent beta level of PHP 5.1, that is PHP 5.1.0.
The Relational DAS uses PDO to access the relational database, and so should run with a variety of different relational databases. At the time of writing it has been tested in the following configurations
MySQL 4.1.14, on Windows. The Relational DAS operates correctly with the php_pdo_mysql driver that comes with the pre-built binaries for PHP 5.1.0.
MySQL 4.1.13, on Linux. It is necessary to have the most up-to-date PDO driver for MySQL, which comes built in to PHP 5.1.0. It may be necessary to uninstall the usual driver that would have come from PECL using pear uninstall pdo_mysql. You will need to configure PHP with the --with-pdo-mysql option.
DB2 8.2 Personal Edition, on Windows. The Relational DAS operates correctly with the php_pdo_odbc driver that comes with the pre-built binaries for PHP 5.1.0.
DB2 8.2 Personal Developer's Edition, on Linux. The Developer's Edition is needed because it contains the include files needed when PHP is configured and built. You will need to configure PHP with the --with-pdo-odbc=ibm-db2 option.
The Relational DAS applies changes to the database within a user-delimited transaction: that is, it issues a call to PDO::beginTransaction() before beginning to apply changes, and PDO::commit() or PDO::rollback() on completion. Whichever database is chosen, the database and the PDO driver for the database must support these calls.