PHP Variable variables

Sometimes it is convenient to be able to have variable variable names. That is, a variable name which can be set and used dynamically. A normal variable is set with a statement such as:

<?php
$a = 'hello';
?>

A variable variable takes the value of a variable and treats that as the name of a variable. In the above example, hello, can be used as the name of a variable by using two dollar signs. i.e.

<?php
$a = 'hello';
?>

At this point two variables have been defined and stored in the PHP symbol tree: $a with contents “hello” and $hello with contents “world”. Therefore, this statement:

<?php
$a = 'hello';
$$a = 'world';
echo "$a ${$a}"; // Hello World
?>

Produces the exact same output as:

<?php
$a = 'hello';
$$a = 'world';
echo "$a $hello";
?>