What does $$ (dollar dollar or double dollar) mean in PHP?

A syntax such as $$variable is called Variable Variable.

For example, if you consider this portion of code :

$real_variable = 'test';
$name = 'real_variable';
echo $$name;

You will get the following output :

test

Here :

• $real_variable contains test
• $name contains the name of your variable : 'real_variable'
• $$name mean "the variable thas has its name contained in $name"
  • Which is $real_variable
  • And has the value 'test'

---

EDIT after @Jhonny's comment :

Doing a $$$ ?
Well, the best way to know is to try ;-)

So, let's try this portion of code :

$real_variable = 'test';
$name = 'real_variable';
$name_of_name = 'name';

echo $name_of_name . '<br />';
echo $$name_of_name . '<br />';
echo $$$name_of_name . '<br />';

And here's the output I get :

name
real_variable
test

So, I would say that, yes, you can do $$$ ;-)

The inner $ resolves the a variable to a string, and the outer one resolves a variable by that string.

So, consider this example

$inner = "foo";
$outer = "inner";

The variable:

$$outer

would equal the string "foo"

It's a variable's variable.

<?php
$a = 'hello';
$$a = 'world'; // now makes $hello a variable that holds 'world'
echo "$a ${$a}"; // "hello world"
echo "$a $hello"; // "hello world"
?>

It creates a dynamic variable name. E.g.

$link = 'foo';
$$link = 'bar';    // -> $foo = 'bar'
echo $foo;
// prints 'bar'

(also known as variable variable)