<?php

  class Singleton1
  {
    private $instance = null;
    
    private function __Construct()
    { }
    
    public function & GetInstance()
    {
      if( $this->instance == null )
        $this->instance = new Singleton1();
      
      return $this->instance;
    }
    
    public function Write( $message )
    {
      print "Singleton1 - " . $message;
    }

  }

  class Singleton2
  {
    private $instance = null;
    
    private function __Construct()
    { }
    
    public function & GetInstance()
    {
      if( $this->instance == null )
        $this->instance = new Singleton2();

      return $this->instance;
    }
    
    public function Write( $message )
    {
      print "Singleton2 - " . $message;
    }

  }
  
  class Main
  {
    public function __Construct()
    {
      Singleton1::GetInstance()->Write( "Singleton1" );
      echo "<br/>";
      Singleton2::GetInstance()->Write( "Singleton2" );
    }
  
  }

  new Main();
?>

If error_reporting(E_ALL) is set, you will see that instead of setting $this in a singleton, $this is set in the Main class. Here is the notice:

Notice: Undefined property: Main::$instance in C:\eclipse\workspace\tmote\tmp_singles.php on line 12

Where line 12 is:
if( $this->instance == null )

Replacing $this->instance with self::$instance and 
private $instance = null;
with,
private static $instance = null;

Will make the code behave as expected.