I think i understand your point of view, but according to the PDO documentation the method expects a string to trigger the logic of named sequences, if it is not a string i think that the logic had to be managed by the parent code.

Informix doesn't support identifiers composed only by numbers or that begin with a number. It is true that if a string of type "1" or even a empty string "" is passed as value the logic of named sequences is triggered but i think that this case is the same as if in a query instead of a table name a numeric string "1" or a empty string "" is used: an error will occur due to try to access to a non-existent database object.