superglobals
Superglobals
Built-in variables that are always available in all scopes
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Several predefined variables in PHP are "superglobals", 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.
These superglobal variables are: $GLOBALS, $_SERVER, $_GET, $_POST, $_FILES, $_COOKIE, $_SESSION, $_REQUEST, $_ENV
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[!NOTE] By default, all of the superglobals are available but there are directives that affect this availability. For further information, refer to the documentation for variables_order.
[!NOTE] Superglobals cannot be used as variable variables inside functions or class methods.
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variable scope, The variables_order directive, The filter extension