<?php

  //You can even add more Dollar Signs

  $Bar = "a";
  $Foo = "Bar";
  $World = "Foo";
  $Hello = "World";
  $a = "Hello";

  $a; //Returns Hello
  $$a; //Returns World
  $$$a; //Returns Foo
  $$$$a; //Returns Bar
  $$$$$a; //Returns a

  $$$$$$a; //Returns Hello
  $$$$$$$a; //Returns World

  //... and so on ...//

?>

<?php
class foo {
    var $bar = 'I am bar.';
    var $arr = array('I am A.', 'I am B.', 'I am C.');
    var $r   = 'I am r.';
}

$foo = new foo();
$bar = 'bar';
$baz = array('foo', 'bar', 'baz', 'quux');
echo $foo->$bar . "\n";
echo $foo->{$baz[1]} . "\n";

$start = 'b';
$end   = 'ar';
echo $foo->{$start . $end} . "\n";

$arr = 'arr';
echo $foo->{$arr[1]} . "\n";

?>

<meta charset="utf-8">
<?php 
$one = "two"; 
$two = "three"; 
$three = "four"; 
$four = "five"; 
echo $$$$one; //prints 'five'. 
?> 

<?php 
$one = "two"; 
$$one = "three"; 
$$$one = "four"; 
$$$$one = "five"; 
echo $$$$one; //still prints 'five'. 
?>

http://php.net/manual/en/language.variables.variable.php#sthash.kYJIhq2A.dpuf