I've thought about this.
How did a catastrophe like PHP come to be so widely used?
I've come up with two answers.
1. Historical accident
There was a need for a PHP-like language to write web back-ends. It could have been Perl, or Python, or PHP, or Ruby, or probably any of a hundred others. At some point, some language gains critical mass. Then everyone uses it because everyone uses it, and we're off to the races. Which language is first to critical mass is--at least somewhat--a matter of chance. As it happens, it was PHP.
2. The default behavior of the language processor is to emit the entire program text on STDOUT.
What this means is that you can take an entire existing static web site--a whole tree of static HTML pages--declare them to be PHP scripts, et voila, there is your web site, just like it was before, except that now you can start adding $variables to your pages and creating dynamic content. It gives managers and salesmen and marketing people and non-programmers a way to get into the game without actually having to do any of that--you know--programming stuff.
Calling this an easy learning curve misapprehends the situation. It is more in the "this one weird trick" category.
The actual PHP learning curve is vertical, which is to say, no one can learn PHP. The language is such a tangled mass of inconsistencies, exceptions, breakage and lossage that acquiring the knowledge necessary to use it competently is virtually impossible.