Why do objects automatically inherit values from another object initiated before or after?

Question

Here Student's class method and variable get affected and present in other object too i.e. $obj1, why does this happen?

class Student {
    public $name;
    public $age;
    public function callme() {
        return 'called';
    }
}

$obj = new Student();
$obj1 = $obj;
$obj->name = 'David';
$obj->age = 12;
echo '<pre>';
print_r($obj);
print_r($obj1);
echo $obj1->callme();

ouput :

Student Object
(
    [name] => David
    [age] => 12
)
Student Object
(
    [name] => David
    [age] => 12
)
called

Answer 1

This behaviour is explained here, when you do the following:

$obj = new Student();
$obj1 = $obj;

$obj1 is actually a reference to $obj so any modifications will be reflected on the original object. If you need a new object, then declare one using the new keyword again (as that's what it's for) as such:

$obj = new Student();
$obj1 = new Student();

(Also, I see @Wizard posted roughly the same thing half way through me writing this but I'll leave this here for sake of examples)

Answer 2

As of PHP 5, $obj and $obj1 hold a copy of the object identifier, which points to the same object. Read http://php.net/manual/en/language.oop5.references.php