Yes, ant CMS is not simple, certainly not for a beginner in PHP.
And it's really too big a subject to cover in a single topic, though you may get some tips on getting started and most likely will have lots more specific questions along the way.
It may be best to find a good tutorial or book on the subject. But do be warned that there is a lot of out-dated and poor quality content out there which can catch out the beginner. These days anyone can write and publish on-line a PHP tutorial, regardless of whether they know what they are doing.
I only say this because I see so many beginners come here learning php from the wrong places and using dreadfully out-dated and insecure code.
As a rule of thumb, avoid anything that uses the obsolete
mysql functions, you should be using
PDO to connect with your database.
Another tell-tale is outdated or poorly constructed HTML, as it generally goes hand in hand with PHP of a similar nature. I regularly see bad php along with bad html, things like
align attributes, use of html for spacing, Eg,
<br /><br /> or
<p> </p> If you see any stuff like that, run a mile and find something else.
Which brings us to:-
If things are being done properly, the logical, processing code will be kept as far as practical, separate from the html code that displays the page. So in it's simplest form, you may have a controller script plus an html template file for a webpage (though it can get more complex).
But the point being, if what you are shown is a real jumble of a lot of PHP logic, mixed in with HTML code, it is likely another poor example of how to do things.