Get (part of) the query string.
The type parameter is either one of the HttpQueryString::TYPE_* constants or a type abbreviation like "b" for bool, "i" for int, "f" for float, "s" for string, "a" for array and "o" for a stdClass object.
key
key of the query string param to retrieve
type
which variable type to enforce
defval
default value if key does not exist
delete
whether to remove the key/value pair from the query string