PHP files are just 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 accessed and the resulting output is
written as HTML within the Web page. When a user accesses a PHP page,
his Web browser only gets sent the HTML code, since the Web server has
processed the PHP code in the background. Most PHP pages are processed
so quickly that it does not noticeably slow down the loading of the Web
page.
The .php extension is important, since it tells the Web server that
the page may include PHP code. Therefore, it must be run through the
server's PHP engine before being sent to a client's Web browser. This
allows dynamic content to be generated each 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 current date and time, data from form
fields submitted by a user, or information from a database. Still,
once the page reaches the user's Web browser, everything is formatted as
HTML.
