The main reason many people can use PHP are sites which provide free PHP scripts. Everyone can install a forum in just a few clicks. The only things you need are a webhost which support PHP, mostly MySQL and a program which can upload the files for you, the so called FTP program. But not every site support every PHP command. Some are, because of security reasons, disabled. This makes it difficult for PHP newbies. They don't know how to change and what to change to the script. So to use most scripts you do need some knowledge of PHP. And not every script is provided with the proper documentation, and without the documentation most users don't even know how to use the include command. Also, some scripts are built by people who can't use proper English, like some French people, or even German people.
But if you want to program scripts yourself you have to learn PHP, and that isn't easy. I can assure you that. First you start with simple things, like variables. That's not so hard, but then you go to preg_match and other stuff. Later you learn how to open documents and edit them with a script. Finally you will learn MySQL, which have some commands too. So basically you need to learn PHP and MySQL. If you think that's all, you're wrong. After these things you want to make your site more dynamic. That's when the real trouble comes. You know those numbers which show you the page? Well, if you think that's simple to make, then you're absolutely wrong. You first need to see how many pages there are, then you need to devide them by the ammount of... you know what I mean.
And still there are thousands of PHP scripts. Is PHP difficult, or is it just our imagination? Are all those commands really usefull for most scripts, or not? It's up to you to decide.