Dynamic PHP variable

PHP allows you to use dynamic variable names, called variable variables. You can name a variable with the value stored in another variable. That is, one variable contains the name of another variable.

$Bangladesh = 147570;
$dhaka = 11500;
$cityname = "Bangladesh";
echo "The size of $cityname is ${$cityname}";
$cityname = "dhaka";
echo "The size of $cityname is ${$cityname}";

The output from this code is

The size of Bangladesh is 147570
The size of dhaka is 11500

Ref: https://www.dummies.com/programming/php/how-to-use-php-variable-variables/