unserialize in PHP

unserialize — Creates a PHP value from a stored representation

Syntax: 
mixed unserialize ( string $str )

unserialize() takes a single serialized variable and converts it back into a PHP value.
Parameters: 

str

    The serialized string.

    If the variable being unserialized is an object, after successfully reconstructing the object PHP will automatically attempt to call the __wakeup() member function (if it exists).

        Note: unserialize_callback_func directive

        It's possible to set a callback-function which will be called, if an undefined class should be instantiated during unserializing. (to prevent getting an incomplete object "__PHP_Incomplete_Class".) Use your php.ini, ini_set() or .htaccess to define 'unserialize_callback_func'. Everytime an undefined class should be instantiated, it'll be called. To disable this feature just empty this setting.

Return Values:

The converted value is returned, and can be a boolean, integer, float, string, array or object.

In case the passed string is not unserializeable, FALSE is returned and E_NOTICE is issued.
Changelog:
Version Description
5.6.0 Manipulating the serialised data by replacing C: with O: to force object instantiation without calling the constructor will now fail.
Examples:

Example #1 unserialize() example
<?php
// Here, we use unserialize() to load session data to the
// $session_data array from the string selected from a database.
// This example complements the one described with serialize().

$conn = odbc_connect("webdb", "php", "chicken");
$stmt = odbc_prepare($conn, "SELECT data FROM sessions WHERE id = ?");
$sqldata = array($_SERVER['PHP_AUTH_USER']);
if (!odbc_execute($stmt, $sqldata) || !odbc_fetch_into($stmt, $tmp)) {
    // if the execute or fetch fails, initialize to empty array
    $session_data = array();
} else {
    // we should now have the serialized data in $tmp[0].
    $session_data = unserialize($tmp[0]);
    if (!is_array($session_data)) {
        // something went wrong, initialize to empty array
        $session_data = array();
    }
}
?>

Example #2 unserialize_callback_func example
<?php
$serialized_object='O:1:"a":1:{s:5:"value";s:3:"100";}';

// unserialize_callback_func directive available as of PHP 4.2.0
ini_set('unserialize_callback_func', 'mycallback'); // set your callback_function

function mycallback($classname) 
{
    // just include a file containing your classdefinition
    // you get $classname to figure out which classdefinition is required
}
?>
Notes:
Warning

FALSE is returned both in the case of an error and if unserializing the serialized FALSE value. It is possible to catch this special case by comparing str with serialize(false) or by catching the issued E_NOTICE.
Warning

Do not pass untrusted user input to unserialize(). Unserialization can result in code being loaded and executed due to object instantiation and autoloading, and a malicious user may be able to exploit this. Use a safe, standard data interchange format such as JSON (via json_decode() and json_encode()) if you need to pass serialized data to the user.