display_db_query：未知列

I'm trying to query a database and display it on a webpage. The current query is:

$query_string = 'SELECT * FROM charContracts WHERE `type` = courier  ORDER BY  `charContracts`.`status` DESC';

I ahve also tried this:

$query_string = 'SELECT * FROM charContracts WHERE `charContracts`.`type` = courier  ORDER BY  `charContracts`.`status` DESC';

Currently these both give me the below error:

display_db_query:Unknown column 'courier' in 'where clause'

I'm using this in phpyadmin but when i try to copy this in a get a syntax error with php.

SELECT * 
FROM charContracts
WHERE  `charContracts`.`type` =  'courier'
ORDER BY  `charContracts`.`status` DESC

Can anyone tell me where i'm wrong. (For Reference.. charContracts is the table, type is a column and courier is one of the choices)

Put "courier" in quotes so that it is interpreted as a string. Now it is interpreted as a column name which is obviously not there.