Zend FR

Tout est dans la source :

* This class simply holds a string, so that fragments of SQL statements can be
* distinguished from identifiers and values that should be implicitly quoted
* when interpolated into SQL statements.
*
* For example, when specifying a primary key value when inserting into a new
* row, some RDBMS brands may require you to use an expression to generate the
* new value of a sequence.  If this expression is treated as an identifier,
* it will be quoted and the expression will not be evaluated.  Another example
* is that you can use Zend_Db_Expr in the Zend_Db_Select::order() method to
* order by an expression instead of simply a column name.
*
* The way this works is that in each context in which a column name can be
* specified to methods of Zend_Db classes, if the value is an instance of
* Zend_Db_Expr instead of a plain string, then the expression is not quoted.
* If it is a plain string, it is assumed to be a plain column name.

PDO n'a aucune connaissance de Zend_Db_Expr. Ce sont les classes Zend_Db_* qui l'utilisent.