public function getVariable($callId, $variableIdentifier)
    {
        $sql = 'SELECT * FROM ' . self::VARIABLES_TABLENAME . '
        WHERE (' . self::CALL_ID_ITEM_COLUMN . ' = ? OR ' . self::CALL_ID_TEST_COLUMN . ' = ?)
        AND ' . self::VARIABLE_IDENTIFIER . ' = ?';

        $params = array($callId, $callId, $variableIdentifier);
        $variables = $this->getPersistence()->query($sql, $params);

        $returnValue = array();

        // for each variable we construct the array
        foreach ($variables as $variable) {
            $returnValue[$variable[self::VARIABLES_TABLE_ID]] = $this->getResultRow($variable);
        }
        return $returnValue;

    }