"php object assignment vs cloning" Code Answer

Objects are abstract data in memory. A variable always holds a reference to this data in memory. Imagine that $foo = new Bar creates an object instance of Bar somewhere in memory, assigns it some id #42, and $foo now holds this #42 as reference to this object. Assigning this reference to other variables by reference or normally works the same as with any other values. Many variables can hold a copy of this reference, but all point to the same object.

clone explicitly creates a copy of the object itself, not just of the reference that points to the object.

$foo = new Bar;   // $foo holds a reference to an instance of Bar
$bar = $foo;      // $bar holds a copy of the reference to the instance of Bar
$baz =& $foo;     // $baz references the same reference to the instance of Bar as $foo

Just don't confuse "reference" as in =& with "reference" as in object identifier.

$blarg = clone $foo;  // the instance of Bar that $foo referenced was copied
                      // into a new instance of Bar and $blarg now holds a reference
                      // to that new instance