PHP - Storing Objects as Strings

Introduction

Objects that you create in PHP are stored as binary data in memory.

PHP provides two functions to help you with this:

Method	Description
serialize()	converts an object - properties, methods, and all - into a string of text
unserialize()	takes a string created by serialize() and turns it back into a usable object

The following example shows these two functions in action:

Demo

<?php
class Person {/*from  ww  w  .  j  ava 2 s.c  om*/
            public $age;
}

$tom = new Person();
$tom->age = 28;
$tomString = serialize($tom);
echo "::::'$tomString'\n";
echo "Converting '$tomString' back to an object... \n";
$obj = unserialize($tomString);
echo "tom's age is: $obj->age \n";
?>

Result

This code creates a simple Person class with one property, $age.

It then creates a new Person object, $tom, and sets its $age property to 28.

It calls serialize() to convert the object to a string, which it displays.

Finally, it converts the string back into a new object,$obj, then displays its $obj->age property (28).

You can actually use serialize() and unserialize() on any PHP value, not just objects.

When you serialize an object, PHP attempts to call a method with the name __sleep() inside the object.

You can use this method to do anything that's required before the object is serialized.

You can create a __wakeup() method that is called when the object is unserialized.

__sleep() is useful for cleaning up an object prior to serializing it, in the same way that you might clean up in a destructor method.

PHP expects your __sleep() method to return an array of names of properties to preserve in the serialized string.

You can use this to limit the number of properties stored in the string.

Demo

<?php
        class User {
          public $username;
          public $password;
          public $loginsToday;

          public function __sleep() {
             // (Clean up; close database handles, etc)
             return array("username","password");
          }/*from   w  w w. j av a  2s .  co m*/
        }
        $user = new User;
        $user->username ="tom";
        $user->password ="mypassword";
        $user->loginsToday = 3;
        echo "The original user object: \n";
        print_r($user);
        echo "Serializing the object... \n";
        $userString = serialize($user);
        echo "The user is now serialized in the following string: \n";
        echo "$userString \n";
        echo "Converting the string back to an object... \n";
        $obj = unserialize($userString);
        echo "The unserialized object: \n";
        print_r($obj);
        echo "\n";
?>

Result

Here, we don't care about preserving the number of times the user has logged in today, so the __sleep() method only returns the "username" and "password" property names.

When the object is restored from the string, the $loginsToday property is empty.

To preserve all your object's properties, use the built-in get_object_vars() function to get an associative array of all the properties in the object.

Then use the array_keys() function to get just the property names as an array, which you can then return from your __sleep() method:

Demo

<?php
        class User {
          public $username;
          public $password;
          public $loginsToday;

          public function __sleep() {
              return array_keys(get_object_vars($this));
          }/*from   w w w .  j av  a 2s .  co  m*/
          public function __wakeup() {
            echo "wakeup \n";
          }
         }

        $user = new User;
        $userString = serialize($user);
        $obj = unserialize($userString);
?>

Result

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