Flette forespørgelser

Hvorfor giver denne flettede forespørgsel en fejl?:

$sql = \"SELECT
companies.Name,
companies.Address,
companies.Postcode,
companies.Phone,
companies.Fax,
companies.WWW,
companies.Email,
postcodes.Postcode,
postcodes.City
FROM companies LEFT JOIN postcodes ON postcodes.Postcode = companies.PostCode where companies.CompanyKey = (select CompanyKey from categorycompanylinks where CategoryKey = \'$firma_cat\')\";

Et eller andet går da helt galt der!...

mysq_error() giver dette:
You have an error in your SQL syntax near \'select CompanyKey from categorycompanylinks where CategoryKey = \'56\')\'