A measre of security

Hi! This is a kind of security-question.... I'm starting up a system, where several customers should be able to login with there own information. Each company I give unique identity with "hard-coding" and each company has its own folder... Harding-coding, something like: $company -> setIDCompany(1); $company -> setIDCompany(2); $company -> setIDCompany(3); Diffrent folders: customers/company1 customers/company2 customers/company3 and so on... This isn't complicated, but when I add a new company.... ...I must be 200% sure that the IDCompany is set correct (There is a chance of setting the wrong ID for a new company or forgetting to change it) The companys ARE NOT ALLOWED to see each others information. Is there any good way of solving this with some sort of comparision-mechanism or something like that? Any thoughts? Best regards /Gustav Wiberg