Dynamic Variable

A dynamic variable is a variable whose name stored in a variable or an expression.

They are identified by a double $ sign, though the same operator may be used multiple times.

They are also called variable variable.

<?php

$a = 'b';
$b = 'c';
$c = 'd';

echo $$$a;
echo $$b;
echo $c;

?>

Documentation

See also The Dangers of PHP’s $$

Related : Variables