Superglobals — Built-in variables that are always available in all scopes
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:
Note: Variable availability
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: Variable variables
Superglobals cannot be used as variable variables inside functions or class methods.