I studied lots of PHP because it was relevant to my general web use, but I noticed some huge learning gaps. This I believe is due to learning procedural pretty good then jumping on the OOP bandwagon things got distorted for me. I will keep this short but if you like the reusability of OOP style code then you most likely will want to use PDO to setup your database queries. This pushed me past a huge sticking point, let me list what I did all together recently.
1. PDO database setup from scratch, learn it by making a few different connection setups. Doing this really helps you learn OOP style coding, plus PDO isn't very hard at all, try-catch is pretty sweet too.
2. SQL commands, I installed SQL on my Mac and from the command line do all my queries for practice, this will up your skills more than any thing. Knowing how to write the actual command from scratch will make coding it a breeze because you know the syntax and whats going on.
3. I wrote a visitors tracking script PHP PDO, many will say just parse server logs, but I wanted something fast I could use on the site for various functions and information purposes. My server logs delete every so often via the servers setup, so keeping specific important data is better this way too.
4. I wrote all my learning into notes so I can refer to them, the most important things or ones I will most likely forget.