Description:
------------
Sometimes statements for pgsql may contain operators that contain symbols '?' or '::', would be really great to have an option like Perl `pg_placeholder_dollaronly` and not to treat those symbols as placeholders.

Test script:
---------------
Create some_table in Postgres:

CREATE TYPE some_type AS (
	some_column hstore
);
CREATE TABLE some_table OF some_type;

<?php

$dbh = new PDO("pgsql:dbname=some_database;host=some_host", 'some_user', 'some_password' );
$sth = $dbh->prepare('SELECT * FROM some_table WHERE some_column ? :some_param');
$sth->execute(array(':some_param' => 'some_criteria'));
...

And right here we get of course

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY093]: Invalid parameter number: mixed named and positional parameters'

Expected result:
----------------
For statement like 

SELECT * FROM some_table WHERE some_column ? :some_param

would be great to be able to tell PDO not to parse '?' and ':' as placeholders in some cases