why all the output is dollardollar

Question

frist
...
<?php
$dollar = 'dollar';
$dollar = $dollar.'dollar';
$dollardollar = $dollar ;
echo $dollar ;
?>

second
..
<?php
$dollar = 'dollar';
$dollar = $dollar.'dollar';
$dollardollar = $dollar ;
echo $dollar ;
?>

third
...
<?php
$dollar = 'dollar';
$dollar = $dollar.'dollar';
$dollardollar = $dollar ;
echo$$dollar ;
?>

Answer 1

$var refers to a variable, with a name which is the value of $var. For instance, if $foo = 'bar', $foo refers to variable $bar.

Answer 2

Oops There was a mistake,

First:
$dollar => $dollardollar => 'dollardollar'
Second:
$dollar => 'dollardollar'
Third:
$$dollar => $dollardollar => $dollardollar => 'dollardollar'

Answer 3

Thanks Mahmud Nabil