Variable Variables
Variable variables refer to a feature that allows to use the value of a variable as the name of another variable. It provides a way to dynamically create and access variables based on runtime values.
Variable variables are denoted by the double dollar sign $$ followed by the name of the variable stored in another variable. More dollar signs may be compounded, though it is rare.
<?php
$a = 'b';
$b = 'c';
echo $$b;
?>