Is there a way to view all the methods and properties of classes that are already available in PHP? Included classes that come with PHP extensions like mysqli or PDO? Are they actually somewhere readable as PHP code or are they written in the same language PHP is written in?
PHP is written in C. Some of the PHP functions are methods that were written for the language, while others work as a kind of wrapper, e.i. PHP passes the call to another library.
As Manh already commented, the source code is available at github, but it is not very easy to read, and contains much more than just the methods and classes that are directly available in the PHP language.