If you are going to serialie an object which contains references to other objects you want to serialize some time later, these references will be lost when the object is unserialized.
The references can only be kept if all of your objects are serialized at once.
That means:
$a = new ClassA();
$b = new ClassB($a); //$b containes a reference to $a;
$s1=serialize($a);
$s2=serialize($b);
$a=unserialize($s1);
$b=unserialize($s2);
now b references to an object of ClassA which is not $a. $a is another object of Class A.
use this:
$buf[0]=$a;
$buf[1]=$b;
$s=serialize($buf);
$buf=unserialize($s);
$a=$buf[0];
$b=$buf[1];
all references are intact.