this example could help:
<?php
class A {
 public $testA = 1;
} 
class B {
 public $testB = "class B";
} 
$a = new A;
$b = $a; 
$b->testA = 2;
$c = new B;
$a = $c;
$a->testB = "Changed Class B";
echo "<br/> object a: "; var_dump($a);
echo "<br/> object b: "; var_dump($b);
echo "<br/> object c: "; var_dump($c);
// by reference 
$aa = new A;
$bb = &$aa; 
$bb->testA = 2;
$cc = new B;
$aa = $cc;
$aa->testB = "Changed Class B";
echo "<br/> object aa: "; var_dump($aa);
echo "<br/> object bb: "; var_dump($bb);
echo "<br/> object cc: "; var_dump($cc);
?>