WorkAround for PHP PDO(with libpq V 9.1.4) binding for use of CITEXT?
Locate the namespace inside which the citext type resides:

select nspname from pg_type t join pg_namespace n
  on n.oid=t.typnamespace where typname='citext';


Prepend that namespace (normally, 'public', but it might be different in your case and it might explain the problem) to the cast to citext:

$sql = "SELECT column1 from schema1.column1 where column1 = 'value'::public.citext";


If that solves the problem but in a way you find inelegant, you might reconsider how you're using schemas and search_path: make sure that all your custom types are accessible no matter what.