super.php

<?php

class Wrong {

  public function getX() {
    return $_GET['x'] ?? 'default x';
  }
}

class Right {

  public function __construct(array $params) {
    $this->params = $params;
  }

  public function getX() {
    return $this->params['x'] ?? 'default x';
  }
}

$wrong = new Wrong();
echo $wrong->getX();

$right = new Right($_GET);
echo $right->getX();