SortableInterface

SortableInterface.php

<?php

  interface SortableInterface { 
    public function getSortValues();
  }
  
  class A implements SortableInterface {
    private $order, $price, $id;
    public function __construct($price, $id, $order) {
      $this->id = $id;
      $this->price = $price;
      $this->order = $order;
    }
    public function getSortValues() {
      return [
        'has_price' => null !== $this->price,
        'has_disease_id' => null !== $this->id,
        'order' => - $this->order,
      ];  
    } 
    public function toString() {
      return sprintf("%s %s %s", $this->price?:"NULL", $this->id ?: "NULL", $this->order ?: "NULL");
    }
  }

  function custom_sort(SortableInterface $s1, SortableInterface $s2)
  {
    $values1 = $s1->getSortValues();
    $values2 = $s2->getSortValues();
    do {
      $c1 = (int) array_shift($values1);
      $c2 = (int) array_shift($values2);
      if ($c1 !== $c2) {
        return $c2 - $c1;
      }
    } while (count($values1) && count($values2));
    
    return 0;
  };
  
  $collection = [
   new A(100, 1, 1),
   new A(130, null, 1),
   new A(20, null, 3),
   new A(null, 1, 5),
   new A(null, 3, 3),
   new A(null, 3, 1),
   new A(null, null, 4),
   new A(null, null, 1),
   new A(null, null, null),
  ];
  
  uasort($collection, 'custom_sort');
  
  foreach ($collection as $item) echo $item->toString(), "\n";

W linii 17. rozumiem, że się pomyliłeś i miało być ogólnie "id", a nie "disease_id", mam rację?

w 39. średnik niepotrzebny.