What is the difference between a PHP and HTML Web page?

PHP files are only like html files, but they can include both HTML and PHP code. The PHP code is parsed (or executed) by the web server when the page is permitted & the resulting output is written as HTML within the web page. When a user entrances a PHP page, his web browser just acquires sent the HTML code, since the Web server has processed the PHP code in the background. Most PHP pages are procedure so fast that it does not clearly slow down the loading of the page.

The .php extension is very significant, because it tells the web server that the page can include PHP code. Thus, it should be run during the server's PHP engine earlier than being sent to a client's web browser. This permits dynamic content to be created each and every time the web page is loaded, based on the variables included in the PHP code. For example, PHP pages may load objects such as the present date with time, data from form fields submitted by a user, or details from a database. Still, formerly the page reaches the user's browser, everything is formatted as HTML.

Correct. The php extension on a webpage enables a web server to parse the code as PHP and hence the PHP fragments are executed properly.