diff --git a/phpBB/includes/functions.php b/phpBB/includes/functions.php
index 4bf991ca9e..5c08d457b6 100644
--- a/phpBB/includes/functions.php
+++ b/phpBB/includes/functions.php
@@ -5550,22 +5550,38 @@ function phpbb_to_numeric($input)
 }
 
 /**
-* Convert 3.0 dbms to 3.1 db driver class name
+* Convert either 3.0 dbms or 3.1 db driver class name to 3.1 db driver class name.
+*
+* If $dbms is a valid 3.1 db driver class name, returns it unchanged.
+* Otherwise prepends phpbb_db_driver_ to the dbms to convert a 3.0 dbms
+* to 3.1 db driver class name.
 *
 * @param string $dbms dbms parameter
 * @return db driver class
 */
 function phpbb_convert_30_dbms_to_31($dbms)
 {
+	// Note: this check is done first because mysqli extension
+	// supplies a mysqli class, and class_exists($dbms) would return
+	// true for mysqli class.
+	// However, per the docblock any valid 3.1 driver name should be
+	// recognized by this function, and have priority over 3.0 dbms.
+	if (class_exists('phpbb_db_driver_' . $dbms))
+	{
+		return 'phpbb_db_driver_' . $dbms;
+	}
+
 	if (class_exists($dbms))
 	{
 		return $dbms;
 	}
 
-	if (class_exists('phpbb_db_driver_' . $dbms))
-	{
-		return 'phpbb_db_driver_' . $dbms;
-	}
+	// Additionally we could check that $dbms extends phpbb_db_driver.
+	// http://php.net/manual/en/class.reflectionclass.php
+	// Beware of possible performance issues:
+	// http://stackoverflow.com/questions/294582/php-5-reflection-api-performance
+	// We could check for interface implementation in all paths or
+	// only when we do not prepend phpbb_db_driver_.
 
 	throw new \RuntimeException("You have specified an invalid dbms driver: $dbms");
 }
diff --git a/tests/functions/convert_30_dbms_to_31_test.php b/tests/functions/convert_30_dbms_to_31_test.php
new file mode 100644
index 0000000000..d08bf87f15
--- /dev/null
+++ b/tests/functions/convert_30_dbms_to_31_test.php
@@ -0,0 +1,40 @@
+<?php
+/**
+*
+* @package testing
+* @copyright (c) 2012 phpBB Group
+* @license http://opensource.org/licenses/gpl-2.0.php GNU General Public License v2
+*
+*/
+
+require_once dirname(__FILE__) . '/../../phpBB/includes/functions.php';
+
+class phpbb_convert_30_dbms_to_31_test extends phpbb_test_case
+{
+	public function convert_30_dbms_to_31_data()
+	{
+		return array(
+			array('firebird'),
+			array('mssql'),
+			array('mssql_odbc'),
+			array('mssqlnative'),
+			array('mysql'),
+			array('mysqli'),
+			array('oracle'),
+			array('postgres'),
+			array('sqlite'),
+		);
+	}
+
+	/**
+	* @dataProvider convert_30_dbms_to_31_data
+	*/
+	public function test_convert_30_dbms_to_31($input)
+	{
+		$expected = "phpbb_db_driver_$input";
+
+		$output = phpbb_convert_30_dbms_to_31($input);
+
+		$this->assertEquals($expected, $output);
+	}
+}