When I try to register a variable to a session from within a class all I get is a variable name with no value associated with it. 
I have an initial file which looks like this: 
<? 
class DOG{} 

class USER{ 
  function USER(){ 
  session_start(); 
  $tmp=new DOG(); 
  $tmp->test="howdy"; 
  session_register('tmp'); 
  }
}

$dog=new User(); 
echo "How's about it."; 
?> 

Then on a different page I have: 
<? 
session_start(); 
echo $tmp->test. "<BR>"; 
echo session_id(); 
?> 

the only thing echoed out is the PHPSESSID value. 
when I go and take a look at the session file I see no value: 
>more sess_afb860c13c89fd697ad261ab8372e184 
>!tmp| 

This problem doesn't happen when I attempt to register from outside a class. Here's an example of what DOES work:
file noclass.html: 
<? 
class DOG{ 
var $name; 
} 

$mydog=new DOG(); 
$mydog->name="poochy"; 
session_start(); 
session_register("mydog"); 
echo "session id: ".session_id()."<br>n"; 
echo "My dog's name is".$mydog->name."<br>n"; 
echo "is mydog registered? ".session_is_registered("mydog"); 
?> 

and the the next file noclass2.html: 
<? 
class DOG{ 
var $name; 
} 

session_start(); 
echo "session id: ".session_id()."<br>n"; 
echo "mydog's name is: ".$mydog->name."<br>n"; 
?> 

The output is as follows: 
noclass.html: 

session id: 7713588e3fbce54b0760c8a082fd5cc3 
My dog's name ispoochy 
is mydog registered? 1 

--------------------------------------- 

noclass2.html: 
session id: 7713588e3fbce54b0760c8a082fd5cc3 
mydog's name is: poochy 

So, obviously an object CAN be stored to a session, but what is stored is only the data associated with the object properties. 

I return to my original question- why is it that an object can't be registerd to a session from within a class?