PHP
is a general-purpose open source scripting language, widely used, and especially suitable for web development and that can be embedded within HTML. What distinguishes PHP from something like JavaScript on the client side is that the code runs on the server, generating the HTML that is then sent to the browser.
The browser receives the results of running this script, but does not know what the source code was. You can even configure your web server to process all your HTML files with PHP, and then there's no way for users to tell you what you have up your sleeve
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