PHP: What does it mean ($$)?

The code and output below.
What’s just happend?

[php]
$tst = "hello";
$$tst = "test value";

var_dump( $$tst );
var_dump( $tst );
var_dump ( $hello) ;
die();
[/php]

string(10) "test value" string(5) "hello" string(10) "test value"