<?php
class Base {}
class Base2 {}

if (class_exists("Base")) {
    class Upper extends Base
    {
      function __construct() {echo "Exists".PHP_EOL;}
    }
} else {
    class Upper extends Base2
    {
      function __construct() {echo "Doesn't Exist".PHP_EOL;}
    }
}

$a=new Upper();
?>