"PHP message: PHP Warning: Undefined variable [...] in [...] on line [...]" / PHP undefined variable but it is defined:

"PHP message: PHP Warning: Undefined variable [...] in [...] on line [...]" / PHP undefined variable but it is defined:
Happens in functions when accessing a (normal) global variable, because even just for reading they are not imported.

PHP
function fun(){
  global $var; // fix: reference the variable `$var` in function with `global` keyword
}

PHP
1 2 3	function fun(){ global $var; // fix: reference the variable `$var` in function with `global` keyword }

* How do I fix "Undefined variable" error in PHP?: https://stackoverflow.com/questions/20391807/how-do-i-fix-undefined-variable-error-in-php/20391827#20391827