This is a bit tricky, because “be a WordPress developer” is subjective. And, you’ll have know-it-alls throwing around the “you’re not a REAL WordPress developer unless you…”. Followed by their arbitrary and subjective criteria.
I’m gonna go with, “how much PHP do you need to know to build a WordPress plugin?” That’s a bit more objective.
And, in that case, it’s a lot less than you might think.
I can tell you my story, anyway. The first WordPress plugin I built was something I called “CommunityPress”. This was way back in the day before BuddyPress and even Mingle or any of the other community plugins like this.
I had big plans for it, but at the time, all it did was add a simple profile page and a sidebar widget. The widget basically showed logged in/logged out status and when it was clicked would take you to your profile to edit.
Nothing super fancy.
At that time, I knew very little PHP. I knew simple stuff like echo, custom functions, some array stuff. But, I knew quite a bit more about WordPress, it’s hooking system and its built-in functions.
And, with that I was able to build my plugin.
That plugin helped me get a gig with a WordPress company.
And, that company is still my #1 client to this day.
Anecdotal, I know… but I personally believe it’s more important to dig into WordPress and understand it, at first, than it is to be a PHP genius. I know a lot of really good PHP developers who are terrible WordPress developers because they don’t take the time to learn how to do things the “WordPress Way”.
It has a lot there to support you…
And, you should use it when possible.
Of course, the more PHP you know the more advanced WordPress plugins you’ll be able to build. But, counter to what a lot of the snobs say… I think you can get going a lot faster than you might think.
On a side note, it’s funny Quora suggested this question to me today. I just launched a WordPress plugin course where I teach you this stuff. So, serendipitous, I guess. Anyway, if you want to check it out, it’s here. It teaches you things like custom post types, shortcodes, handling form submissions and the like.
Source: My answer on Quora.com here.