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Cloning Objects

When you assign an object from one variable to another, you do not copy it, but merely assign a handle or reference to it.

<?php

  $a = new SomeClass();
  $b = $a;

  // $a and $b point to the same underlying instance of SomeClass

?>

However, situations will arise where I'll actually want to assign a full copy of an existing object to a new variable. This is known as cloning, and is done via the clone operator.

<?php

  $a = new SomeClass();
  $b = clone $a;

  // $b now is a NEW object with all the member variables set
  // to the values they had in $a

?>

When you clone an object in PHP, the language creates a new instance of the class and assigns copies of the corresponding variables in the original instance to this new object's member variables by default. However, this is a shallow copy, meaning that if one of those member variables is itself a reference to an object, only that reference is copied.

If the default behavior for cloning is not sufficient for your needs, you can implement the __clone function in your class, which PHP calls after it has performed the shallow copy of the member variables. If one of my member variables was a connection to a database, I might want to establish a new connection to the same database when my object is cloned.

<?php

  class CloneDemo
  {
    public $dbconnection;
    public $connect_string;

    public function __clone()
    {
      // $connect_string is copied for us.
      $this->dbconnection = $this->reestablish_connection(
         $this->connect_string);
    }
    // etc.
  }

?>

Now, whenever the object is cloned, the __clone method is called and the connection is also cloned.

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