index.php

<?php

class test
{
    function __construct()
    {
        include("./test.php");
        $class_var_name = "some_var_name";
        $class_name = "extension";
        $this->$class_var_name = new $class_name();
    }
}

$class = new test();
$var_name = "hi";
$class->some_var_name->$var_name = "Hello, World!";

echo $class->some_var_name->hi;

?>