I have created two classes for senior freelancers. PHP and PL/SQL. I think PHP is not very good language but it is popular among freelancers. Follow this class to learn how to quickly build a website.
PL/SQL is specific to Oracle databases. Then why is one of the most useful language for freelancers? It is because learning this language you can create back-end complex business applications. You also learn SQL that can be useful for other databases.