By default, it is important to have your PHP code in a .php file, that means the file name must ended with .php, because .php files are set up on most servers to be read by the system's PHP interpreter. If you add PHP code to say, .html files, it cannot do anything because those files are not set to be parsed (rendered / executed) by the PHP interpreter.
This is pretty basic for PHP programming, but beginners to PHP or web development may forget the coding rules.