Quirks in Sorting Pg_description

The following query works from Psql's prompt but not from Psql's "-c"
command or from PHP's PDO interface.

SELECT description FROM pg_catalog.pg_description WHERE description
SIMILAR TO '\d+' ORDER BY description::integer DESC LIMIT 1;

The result should be the description with the highest integer value.
Instead it appears to be sorting like text even though it has been cast
to an integer.

Anyone know of a workaround?