Where to find PhP's functions implementation? How is implemented engine? How is implemented mechanism of communication between front and backend
https://www.php.net/download-docs.php
https://www.php.net/manual/en/language.variables.superglobals.php
Several predefined variables in PHP are "super-globals", which means they are available in all scopes throughout a script. There is no need to do global $variable; to access them within functions or methods.
$GLOBALS
$_SERVER
$_GET
$_POST
$_FILES
$_COOKIE
$_SESSION
$_REQUEST
$_ENV