If all you want to do is "practice" rather than build actual tools or apps, then maybe you could make a study out of every function.
In other words, depending on what extensions you have installed there might be 1800 - 3000 various functions available, give or take.
Pick a function to learn, give it a quick study, then write some made up tool which makes use of it. This probably sounds crazy but I'm not talking about spending days on a simple little function. I'm saying take a function, look at what it's for and what arguments it takes and what the output and error conditions are. Then figure out a way to test various inputs and read the outputs and use the function in some way.
Simple functions might only be a 10 or 20 minute study, not talking about a lot of time.
Another way is to study functions by their relationship. For example do the above study and practice but for all string manipulation functions, or all regex functions, or all cache functions, or output capturing functions, etc.
If you hit a larger topic, like studying all the mysqli functions or PDO functions, this could take a while, but other classes, not so much, like playing with json encode and decode.
If all you're doing is "practice", then at the least this will give you practice making use of PHP functions and you'll get a general idea and feeling of how they tend to work, how the outputs tend to work, falsey and truthy values, etc.
If you want to practice in a more real world way, solving actual problems, check out the coding challenges on reddit here: https://www.reddit.com/r/dailyprogrammer/
If you are the obsessive compulsive type, you could go all the back to the challenge #1 on the beginner side and just do every one of them. Store each of your answers in a Git repository or something.
Anyway, good luck!