I append my field and data a my query
 $SQL_WHERE .= " AND e.description LIKE :description ";
 $DQL_WHERE[':description']   = "%".$description."%";

 a var_dump of DQL_WHERE
 array(1) { [":description"]=>  string(6) "%toto%" }

 generate SQL :
 SELECT e.id ... FROM table e ... WHERE (1 AND e.description LIKE :description) - params: (undefinedoto%)