variable variables - what is "$$" in PHP

I saw this code

if (is_null($$textVarName)) {
$$textVarName = $_defaultTexts[$type];
}

what is code "$$" ?

Answer

It's evil is what it is.

That will take the value that's in $textVarName and use that as a variable name. For example:

$foo = 'hello';
$hello = 'The Output';
echo $$foo; // displays "The Output"