take the following example:

<?php

class MethodsOnly {
	//var $dummy;
	function doSomething() {
		print("OBJ: doing something\n");
	}
}// class

$OBJ = new MethodsOnly();
$OBJ->doSomething();
print("empty(OBJ):   ".(empty($OBJ) ? "true" : "false")."\n");
print("OBJ == null:  ".(($OBJ == null) ? "true" : "false")."\n");
print("OBJ === null: ".(($OBJ === null) ? "true" : "false")."\n");

?>

it gives the following output:

OBJ: doing something
empty(OBJ):   true
OBJ == null:  true
OBJ === null: false

now, if you uncomment the $dummy member variable:

OBJ: doing something
empty(OBJ):   false
OBJ == null:  false
OBJ === null: false

Sure, one should/could use "static calls" - like MethodsOnly::doSomething() - in this case (class without member vars), but we discovered this when migrating a project from PHP3 to PHP4 
.. and, anyway, shouldn't an existing object behave the same wether it has member variables or not? ;)