My experience in US/Ohio (traditional employment not freelancing) is that good PHP developers with professional qualifications & experience (CS degree, high volume websites, ecommerce, large scale CRM/CMS) are hard to come by. True, the jobs that require this experience aren't nearly as plentiful as the kind you would find on freelance sites but my last three employers were all having difficulty (even at the top of the salary range for devs in this area) finding qualified PHP programmers to maintain and extend their applications.
I can't speak to how much freelancers on those sites make but I can't imagine it's very much. You're dealing with the bottom of the barrel as far as customers are concerned and competing against people who are willing to do the work for a few dollars an hour.
What's more, your potential customers aren't looking for a "good" PHP programmer they're looking for a "cheap" PHP programmer as they see programming as a commodity. Because most of them don't appreciate the difference between poor programming and good programming they're not willing to pay a premium for it.
So, if you're good, I would specialize in something that has a higher perceived than general PHP programming where you can differentiate what you offer from everyone else. Things like payment processing, Wordpress/Drupal/SugarCRM CMS development, or Zend/Symfony framework development, etc.
Look to bring good skills and experience to a market that values them and where you can effectively market them instead of trying to convince people with a $100 budget your skills are better than the guy bidding $15 for the job...