1. <pre>
  2. <?php
  3.  
  4. class S {
  5.  
  6.         protected $_data = array();
  7.  
  8.         public function __construct($separator) {
  9.                 $name = get_class($this);
  10.                 for ($i = 1; $i < 3; $i++) {
  11.                         for ($j = 1; $j < 3; $j++) {
  12.                                 $this->_data[$i][$j] = sprintf('Property %d%s%d of %s', $i, $separator, $j, $name);
  13.                         }
  14.                 }       
  15.         }
  16.  
  17.         public function get($id, $pid) {
  18.                 return isset($this->_data[$id][$pid]) ? $this->_data[$id][$pid] : null; 
  19.         }
  20. }
  21.  
  22. class S1 extends S {
  23. }
  24.  
  25. class S2 extends S {
  26. }
  27.  
  28. abstract class A {
  29.  
  30.         abstract protected function i();
  31.  
  32.         public static function getById($id, $property_id) {
  33.                 if ($id <= 0 || $property_id <= 0) {
  34.                     throw new Exception('Неверные данные');
  35.                 }
  36.                 return static::i()->get($id, $property_id);
  37.         }
  38. }
  39.  
  40. class B extends A {
  41.  
  42.         private static $instance;
  43.  
  44.         protected function i() {
  45.                 if (!self::$instance) {
  46.                         $len = rand(1, 20);
  47.                         self::$instance = new S1(str_repeat('-', $len));
  48.                 }
  49.                 return self::$instance;
  50.         }       
  51.  
  52. }
  53.  
  54. class C extends A {
  55.  
  56.         protected function i() {
  57.                 $len = rand(1, 20);
  58.                 return new S2(str_repeat('*', $len)); 
  59.         }       
  60.  
  61. }
  62. for ($i = 0; $i < 5; $i++) {
  63. ?>
  64. <div style="border:1px #ccc dashed;"><?php var_dump(B::getById(rand(1,2), rand(1,2))); var_dump(C::getById(rand(1,2), rand(1,2)));?> </div>
  65. <?php
  66. }


If there will be "return self::i()->get($id, $property_id);", you will get 

Fatal error: Cannot call abstract method A::i() in /var/web/tests/lsb_2.php on line 37
 
Call Stack:
    0.0003     350596   1. {main}() /var/web/tests/lsb_2.php:0
    0.0003     350728   2. A::getById() /var/web/tests/lsb_2.php:65