I would read as much of the Manual as you can, even if it doesn’t stick - months down the road when you get an error looking like this:
PHP Parse error: syntax error, unexpected T_STRING, expecting T_PAAMAYIM_NEKUDOTAYIM
You might have a noggin’ moment, something like this: “I remember seeing something like that in the manual a very long time ago!”
It happens for me often - where old fragments of text stick in my mind (not the whole thing) from skimming over the manual.
With the hope that this might stick, the T_PAAMAYIM_NEKUDOTAYIM is Hebrew (the official name in the PHP world) for this: “::” the double colon or the scope resolution operator or the static or class-constant reference.
Set a goal for yourself. Define each part of that goal and then set about searching the manual and googling for tutorials on it - that is how I self taught my self. I went through about a gazillion revisions of my original goal, but, here I am employed for it now…
Code Igniter is awesome - and who cares if you don’t know all of PHP if all of this is just a hobby. If you plan to be skilled with it, and possibly making money with it, then do care about knowing how CI works and ultimately about PHP - about how the array keys are turned into variables when passed to Views or how the URI is mapped to your Controller/Method/Parameter or how the Validation library works.
I have only been doing this stuff for 2 years and despite my lack of experience, just taking the time to read the manual and painstakingly debug my code (with print and exit()) has greatly enhanced the fun and possibilities that are open to me.