I believe you that WordPress is a poor choice for learning PHP. I am not learning WordPress to learn PHP, I am learning PHP to learn WordPress.
Note that PHP is a combination of a language and libraries. Much of what are in the libraries are specific to what a server-side web application needs to do. In the IIS environment, ASP.Net is the libraries and C# is a general-purpose language that can be used for many things. Java and JavaScript are also general-purpose. Java can be used in things like DVD and BluRay players. C/C++ can also be used in embedded applications and many other things. The PHP language, however, has dependencies on its use for web applications, right?