<?php

class Label
{
    var $text;
    function setLabel($text)
    {
        $this->text = $text;
    }
}

$foo = new Label();
$bar1 = $foo;
$bar2 =& $foo;

$foo->setLabel("foo");

echo "Original values: all are same\n";
echo "foo:".$foo->text." bar1:".$bar1->text." bar2:".$bar2->text."\n";

$bar1=new Label();
$bar1->setLabel("bar1");

echo "After bar1 was reassigned: notice that foo does not change its value:
foo and bar1 are not the same variable\n";
echo "foo:".$foo->text." bar1:".$bar1->text." bar2:".$bar2->text."\n";

$bar2=new Label();
$bar2->setLabel("bar2");

echo "After bar2 was reassigned: notice that foo changes its value: foo and
bar2 are the same variable\n";
echo "foo:".$foo->text." bar1:".$bar1->text." bar2:".$bar2->text."\n";
?>

PHP 5.2.9 (cli) (built: Apr 15 2009 09:30:33)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2009 Zend Technologies

Original values: all are same
foo:foo bar1:foo bar2:foo
After bar1 was reassigned: notice that foo does not change its value: foo
and bar1 are not the same variable
foo:foo bar1:bar1 bar2:foo
After bar2 was reassigned: notice that foo changes its value: foo and bar2
are the same variable
foo:bar2 bar1:bar1 bar2:bar2

$obj = new StdClass; // this is a copy-on-write
$obj->property = 'hi'; // the property is a reference

$obj2 = $obj; // copy on write

$obj2->property = 'bye'; // $obj->property also changed since it is a reference

$obj2 = null; // $obj2 changed, thus it has to be copied, $obj stays the same

$property = $obj->property; // copy-on-write reference

$property = null; // $property changed, thus copied, does not affect $obj->property;

In fact as far as I can see, all assignment is copy-on-write, even __clone(), unless specifically referenced with & or an object property (that has more then one reference to it).
The notion that PHP5 objects are assigned by reference, or passed by reference is not true, only the properties of that object are. All variables seem to be passed or assigned as copy-on-write unless using &.

Passing is a whole other thing.
function foo( $text ) {
//bla
}
Will copy a string/int/double (scalars).
This is passing something to a function.
in PHP5 the default action is pass by reference, FOR OBJECTS.
So, when you pass an object (to an function or something), then it will make a reference by default.
So just keep in mind that there is a difference between assigning and passing.

so my understanding is that $obj = $obj2 in PHP 5 is the same as in Ruby or Java.