I think that is a good question.
I can show you a list of links but you have probably already read them, so I will spare you.
They all seem to give a nice 30,000 feet view of PHP, its general abilities and pros/cons of use and so on.
However it is not until you start to use those functions you talk about that you start to get those "ding" moments where suddenly the ground shifts and your horizons suddenly veer away (or come sharply into focus).
Then you suddenly get a mole's eye view of PHP, as you are grappling with code which is essentially talking to the Operating System and also talking over Networks on your behalf.
So you write something as simple as file_get_contents('filenamehere.txt') and what is happening is that PHP, through compiled C code is ready to perform the kind of things you use your windows file manager for, you have opened a file and read it into memory.
So despite this caveat that I am talking in general terms and with my tin hat on in readiness, let me try and explain it like this for you.
You know your desktop computer, the one you are probably typing on now, well most of the basic things it can do, PHP can tell it to do - in one way or another.
The result of that work can be directed to appear as text in a html stream which is sent back to the browser.
That is the top and bottom of it.
Now, how you get on from here depends largely on how well you know computers.
Have you done any programming? How well do you know your OS?
In my formative PHP years, where my knowledge of "what computers can do" was really quite sketchy (I have no formal IT training) I joined a site which still exists called "PHPClasses", which probably still is full of fairly annoying self-promotion, however I got a weekly email containing all of that weeks new classes which had been added. I found this a glorious way to have drip fed into me the kind of weird and wonderful things PHP is capable of being used for.
PHP latest classes RSS feed.
How these things are achieved will mean reading the sometimes scant descriptions, but mainly will involve downloading the code and picking your way through it. But even if you don't, it is a great way to add the font of knowledge which is comes under the heading "What can PHP do?"
This will require a knowledge of how to program. So for you its rather a chicken and egg situation.
The other site that also opened my eyes is Sourceforge.net, do a search, constrain it to the language PHP and see the kind of applications which are being created using PHP.
The big, big caveat I have to now make is that some of the code you find may well not be well written, at all. But you will not be fit to make that judgement or close the security loopholes, or tidy it up, or bend it to your will until you teach yourself how to program. Also, read the licences carefully before using the code you find on any site. They cannot stop you using the ideas though.
Hope this POV helps you, and good luck.