The document discusses six extra skills that every PHP developer should have to succeed as an independent contractor. The skills are: knowledge of JavaScript, HTML5 and CSS3 for full-stack web development; understanding what you don't know to accurately assess project bids; strong business communication skills; understanding basic business finances; project management abilities; and networking with other developers and past clients. Mastering these additional skills can help PHP developers compete and find more work in the growing freelance market.