Скачать или смотреть How PHP handles variable variables

PHP uses dynamic variable name called variable variables.

In case of variable variables, a variable name is created by the value of another variable.

Let's define a variable and assign it with a value.
$travel = "car";

Now, create a variable variables. Here, first $ sign is the traditional one to create a variable in PHP. Next, $travel is the variable we have created earlier. When these two combined, it becomes $car, because variable $travel contains the value car. So, the variable becomes $car = "Green".
$$travel = "Green";

Finally, if we want to get the value Green we need to display $car. This is the power of variable variables.