Strange behavior adding this object to this static array

Question

At the moment I test with this piece of code:

<?php
    class Alert {
        private $type;
        private $message;
        public static $_alerts = array();

        public function add($type, $message) {
            $this->type = $type;
            $this->message = $message;
            self::$_alerts[] = $this;
        }
    }

    $alert = new Alert();

    $alert->add("warning", "test 1");
    $alert->add("error", "test 2");

    echo "<pre>";
    print_r(Alert::$_alerts);
    echo "</pre>";

But my results are not like expected:

Array
(
    [0] => Alert Object
        (
            [type:Alert:private] => error
            [message:Alert:private] => test 2
        )

    [1] => Alert Object
        (
            [type:Alert:private] => error
            [message:Alert:private] => test 2
        )

)

Why is my added object changed?

Test area: http://codepad.viper-7.com/6q2H2A

Answer 1

That's because your object (i.e. $this in internal context) will be copied by reference, not by value. To do copy by value, you'll need to do:

    public function add($type, $message) 
    {
        $this->type = $type;
        $this->message = $message;
        self::$_alerts[] = clone $this;
    }

As an alternative, you'll need to instantiate (that is, for example, constructions like new self - but clone seems to be more flexible here) your object as many times as you'll wish to copy.

By the way, there's easy way to realize what's going on. Use var_dump() instead of print_r() - then you'll see that objects are actually same. Sample for your code (i.e. where copying is not fixed yet):

array(2) {
  [0]=>
  object(Alert)#1 (2) {
    ["type":"Alert":private]=>
    string(5) "error"
    ["message":"Alert":private]=>
    string(6) "test 2"
  }
  [1]=>
  object(Alert)#1 (2) {
    ["type":"Alert":private]=>
    string(5) "error"
    ["message":"Alert":private]=>
    string(6) "test 2"
  }
}

-as you can see, objects are same there.