The $a is a variable with the $a name that stores the value like string, int, float, etc.
The $$aa is a reference variable that stores the value of the $variable inside it.
Examples.
<?php
$a = "xyz";
$$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.
