The $a is a variable with the $a name that stores the value like string, int, float, etc.
The $$aa is a reference variable that stores the value of the $variable inside it.
<?php $a = "xyz"; $$b = 50; echo $a." "; echo $$b." "; echo $xyz; ?>
In the above example, we have assigned a value to the variable a as xyz. The value of the reference variable $$b is assigned as 50.
Now we have printed the values $a, $$b and $xyz.