Whenever someone is new and asks any advice about where to start on their path to learning PHP. In addition to the documentation. Two places I usually recommend are: PHPTheRightWay and PHP-FIG.
With front end frameworks today, such as bootstrap. It's a lot easier to get going with a decent looking site. Unfortunately a clean front-end, does not automatically translate to clean back-end.
When working with a lot of 3rd party software. Efficiency will be left out for this topic. I want to focus solely on variable names and readability.
Lets look at two code samples
Software A:
if($x == $y){
return z();
}
At a glance, a person looking at this code for the first time will have no idea what $x, $y and $z are doing. As the author. Coming back to this code at a later time. It's also highly likely you will no longer remember the intent.
Software B:
if ($usersRole == $moderator) {
return moderatorControlPanel();
}
By giving descriptive variable and function name. The intent is more obviously.
These are basic examples, but the point is readability goes a long way.