$_string : string
SQL literal string.
Horde_Rdo literal query string object.
If you need to pass a string that should not be quoted into a Horde_Rdo_Query object, wrap it in a Horde_Rdo_Query_Literal object and it will not be quoted or escaped. Note that of course you need to be very careful about introducing user input or any other untrusted input into these objects.
Example: $literal = new Horde_Rdo_Query_Literal('MAX(column_name)');