Q:

PHP what does it mean when using this '$$var'?

I am trying to learn php, and I saw this in a foreach loop what does it mean? 
I understand &$var which its a direct reference to the memory address of the object.
But what does $$var means? what is it exactly?
This is the example.
    foreach($this->vars as $key => $value)
    {
        $$key = $value;
        echo "$$Key: " . $$key;
        echo "Key: " . $key;
        echo "<br/>";
        echo "Value: " . $value;
    }

A:

You're looking at a variable variable. e.g.
// original variable named 'foo'
$foo = "bar";

// reference $foo dynamically by evaluating $x
$x = "foo";
echo $$x; // "bar";
echo ${$x}; // "bar" as well but the {} allows you to perform concatenation

// different version of {} to show a more "complex" operation
$y = "fo";
$z = "o";
echo ${$y . $z}; // "bar" also ("fo" . "o" = "foo")

To show an example more closely matching your question:
$foo = "foo";
$bar = "bar";
$baz = "baz";

$ary = array('foo' => 'FOO','bar' => 'BAR','baz' => 'BAZ');
foreach ($ary as $key => $value){
  $$key = $value;
}

// end result is:
// $foo = "FOO";
// $bar = "BAR";
// $baz = "BAZ";

