Magic explained for: http://3v4l.org/jNb0R

going_crazy.php

<?php

// assign the value foo to a variable named x
$x = "foo";
// assign the value bar to a variable named "x", the name includes the " ;)
${'"x"'} = "bar";
// assign the value of a variable, whos name you determine from the content
// of the HEREDOC, which is "x" (the name includes the "), to the variable named doc
$doc = "${<<<'HERE'
"x"
HERE
}";
// print the value and type of variable doc
var_dump($doc);