Well, I have an irrelevant college degree and I'm doing exactly this at 28 yrs of age. It's been about 4-5 months and I've done enough in this period to join a web development company as an intern. So, YES, it can be done if you're passionate about it.

You can check out my blog for some ideas on what to build/read to get started. I started with Python though not PHP. pragmaticstartup.wordpress.com