Find a project that you enjoy and start implementing it. The more you do it the better you get. There are always lots of interesting blog posts over at
Planet-PHP for example but unless you practice you won't get any better.
It takes time and the usual trial-and-error process to find the best techniques and people who are worth to be followed. The problem with PHP is that the barrier of entry is too low so most of the devs are far from being experts. They don't care about improving themselves; they even don't take the time to read the relevant sections of the manual. It's very hard to find good PHP developers even today. Ask a web agency they'll tell you that.
Just keep doing it and one day you'll notice that you became a professional PHP developer.
I would say it takes 5 years of active day-to-day involvement with PHP to become a pro PHP dev but it depends on your previous (web) development experiences of course.