what is "$$" in PHP

It's evil is what it is.

That will take the value that's in $textVarName and use that as a variable name. For example:

$foo = 'hello';
$hello = 'The Output';
echo $$foo; // displays "The Output"

foreach($_POST as $key=>$value)$$key=$value;

now, automagically, if the previous form had a field named 'username' you now have a variable called $username that holds the value submitted in the form. not the greatest or secure method, but when you have a pocket full of nails, this is a heck of a hammer

this is pretty bad practice and is never encouraged but all PHP coders I know secretly sorta like it.

For reference, see: http://php.net/manual/en/language.variables.variable.php