The SplObserver interface

<?php
/**
 * Subject,that who makes news
 */
class Newspaper implements \SplSubject{
 private $name;
 private $observers = array();
 private $content;
 
 public function __construct($name) {
 $this->name = $name;
 }
 //add observer
 public function attach(\SplObserver $observer) {
 $this->observers[] = $observer;
 }
 
 //remove observer
 public function detach(\SplObserver $observer) {
 
 $key = array_search($observer,$this->observers, true);
 if($key){
 unset($this->observers[$key]);
 }
 }
 
 //set breakouts news
 public function breakOutNews($content) {
 $this->content = $content;
 $this->notify();
 }
 
 public function getContent() {
 return $this->content." ({$this->name})";
 }
 
 //notify observers(or some of them)
 public function notify() {
 foreach ($this->observers as $value) {
 $value->update($this);
 }
 }
}
/**
 * Observer,that who recieves news
 */
class Reader implements SplObserver{
 private $name;
 
 public function __construct($name) {
 $this->name = $name;
 }
 
 public function update(\SplSubject $subject) {
 echo $this->name.' is reading breakout news <b>'.$subject->getContent().'</b><br>';
 }
}
$newspaper = new Newspaper('Newyork Times');
$allen = new Reader('Allen');
$jim = new Reader('Jim');
$linda = new Reader('Linda');
//add reader
$newspaper->attach($allen);
$newspaper->attach($jim);
$newspaper->attach($linda);
//remove reader
$newspaper->detach($linda);
//set break outs
$newspaper->breakOutNews('USA break down!');
//=====output======
//Allen is reading breakout news USA break down! (Newyork Times)
//Jim is reading breakout news USA break down! (Newyork Times)

Beware, you have written :
 if($key){
 unset($this->observers[$key]);
 }
When this should be :
 if(false !== $key){
 unset($this->observers[$key]);
 }
If the observer you want to delete is the first in your array, you will never delete it because the key would equal 0 and 0 == false as you know.