Question

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what is Object Cloning in php?

I am studying a existing framework which uses a “clone” keyword a lot, not sure whether this is a good idea to do this ? i dont really understand the need to use the ‘clone’ keyword.

for example have look at this coding

i.e

  public function getStartDate ()
  {
    return clone $this->startDate;
  }

to me this function should be like below, i dont see the need of the clone.

  public function getStartDate ()
  {
    return $this->startDate;
  }

Answer 1

Reason for using clone is that PHP when working with object always returns object as a reference, not as a copy.

That is why when passing object to a function you don’t need to specify it with & (reference):

function doSomethingWithObject(MyObject $object) { // it is same as MyObject &object
   ...
}

So in order to get object copy you have to use clone keyword
This is an example on how objects are handled by php and what clone does:

class Obj {
    public $obj;
    public function __construct() {
        $this->obj = new stdClass();
        $this->obj->prop = 1; // set a public property
    }
    function getObj(){
        return $this->obj; // it returns a reference
    }
}

$obj = new Obj();

$a = $obj->obj; // get as public property (it is reference)
$b = $obj->getObj(); // get as return of method (it is also a reference)
$b->prop = 7;
var_dump($a === $b); // (boolean) true
var_dump($a->prop, $b->prop, $obj->obj->prop); // int(7), int(7), int(7)
// changing $b->prop didn't actually change other two object, since both $a and $b are just references to $obj->obj

$c = clone $a;
$c->prop = -3;
var_dump($a === $c); // (boolean) false
var_dump($a->prop, $c->prop, $obj->obj->prop); // int(7), int(-3), int(7)
// since $c is completely new copy of object $obj->obj and not a reference to it, changing prop value in $c does not affect $a, $b nor $obj->obj!