For clarification, this isn't checking if the value is the default value, it's checking if the property is. The description wasn't as clear as the notes in return. For example:
<?php
class A{
public $foo = 'bar';
}
$a = new A();
$a->foo = 'nope';
$a->new_prop = 'Hello World';
// assume $a_ref_* are your reflection property classes
echo $a_ref_foo->isDefault(); // TRUE even though we changed the value, the property is a default property
echo $a_ref_new_prop->isDefault();// FALSE we added the new_prop property, it's not default
?>