Best I can suggest is to network: get to know people in any part of the business, so that when someone needs a PHP developer, you may pop into their mind. Look for local user group meetings and other occasions to personally meet both customers and others doing what you want to do (they can be both a source of information and a source of jobs if they refer you to a client when they don't have the time to take on a new job).
If you're in the US, western Europe, and other high cost of living countries, you may find the assorted freelancer "auction" sites of questionable worth, as you'll be competing against low bidders from low cost of living countries, as well as kids looking for a little video game money, etc. I really think you're better off trying to contact local people. You could even do some gratis work for your local church, charity, school, etc. just to get your name out there.