Just some reminder which may save somebody some time regarding the `$options` array: 

Say you want to be on the safe side and not allow any objects to be unserialized... My first thought was doing the following:

<?php
$lol = unserialize($string, false);
// This will generate:
// Warning: unserialize() expects parameter 2 to be array, boolean given
?>

The correct way of doing this is the following:
<?php
$lol = unserialize($string, ['allowed_classes' => false]);
?>

Hope it helps somebody!

Keep in mind that the allowed_classes does not use inheritance, i.e. allowing an interface is not possible and sub-classes won't pass the check. See https://3v4l.org/tdHfl

Just a note - if the serialized string contains a reference to a class that cannot be instantiated (e.g. being abstract) PHP will immediately die with a fatal error. If the unserialize() statement is preceded with a '@' to avoid cluttering the logs with warns or notices there will be absolutely no clue as to why the script stopped working. Cost me a couple of hours...

Here's a simple function to get the class of a serialized string (that is, the type of object that will be returned if it's unserialized):

<?php
function get_serial_class($serial) {
    $types = array('s' => 'string', 'a' => 'array', 'b' => 'bool', 'i' => 'int', 'd' => 'float', 'N;' => 'NULL');
    
    $parts = explode(':', $serial, 4);
    return isset($types[$parts[0]]) ? $types[$parts[0]] : trim($parts[2], '"'); 
}
?>

I use this when saving a serialized object to a cookie, to make sure it is the right type when I go to unserialize it.

The type names are the same format/case as you would see if you did a var_dump().

For the people who are getting the error 

PHP Notice:  unserialize(): Error at offset 191 of 285 bytes in ...

and are getting the data from a database, Make sure that you have the database set the the correct encoding, I had the database set as latin1_swedish_ci and all of the data looked perfect, Infact when i copied it into a online unserialize it worked fine. I changed the collation to utf8mb4_unicode_ci and all worked fine.

Talk on Exploiting PHP7 Unserialize here: https://media.ccc.de/v/33c3-7858-exploiting_php7_unserialize

In the Classes and Objects docs, there is this: In order to be able to unserialize() an object, the class of that object needs to be defined.

Prior to PHP 5.3, this was not an issue.  But after PHP 5.3 an object made by SimpleXML_Load_String() cannot be serialized.  An attempt to do so will result in a run-time failure, throwing an exception.  If you store such an object in $_SESSION, you will get a post-execution error that says this:

Fatal error: Uncaught exception 'Exception' with message 'Serialization of 'SimpleXMLElement' is not allowed' in [no active file]:0 Stack trace: #0 {main} thrown in [no active file] on line 0

The entire contents of the session will be lost.  Hope this saves someone some time!

<?php // RAY_temp_ser.php
error_reporting(E_ALL);
session_start();
var_dump($_SESSION);
$_SESSION['hello'] = 'World';
var_dump($_SESSION);

// AN XML STRING FOR TEST DATA
$xml = '<?xml version="1.0"?>
<families>
  <parent>
    <child index="1" value="Category 1">Child One</child>
  </parent>
</families>';

// MAKE AN OBJECT (GIVES SimpleXMLElement)
$obj = SimpleXML_Load_String($xml);

// STORE THE OBJECT IN THE SESSION
$_SESSION['obj'] = $obj;

__PHP_Incomplete_Class Object Demystified

1. First take note of the output. A simple example:

__PHP_Incomplete_Class Object (
[__PHP_Incomplete_Class_Name] => SomeObject1
[obj1property1] => somevalue1 [obj1property2] => __PHP_Incomplete_Class Object ( [__PHP_Incomplete_Class_Name] => SomeObject2 [obj2property1] => somevalue1 [obj2property2] => Array (
['key1'] => somevalue3, ['key2'] => somevalue4 ) ) )

2. We analyze this and break it down. 
__PHP_Incomplete_Class Object tells you there is an object that needs to be declared somehow. 
__PHP_Incomplete_Class_Name simply tells you the expected class name. It is just one of the properties for now.

So we have:
a) an unknown object that has a class name SomeObject1 (first class)
b) it has 2 properties, namely obj1property1 and obj2property2
c) obj2property2 is itself an object whose class name is SomeObject2 (the second class)
d) SomeObject2 has two properties, obj2property1 and obj2property2
e) obj2property2 is an array that contains two elements

3. Now that we have an idea of the structure, we shall create class definitions based from it. We will just create properties for now, methods are not required as a minimum.

<?php
class SomeObject1 {
        public $obj1property1;
        public $obj1property2;
}
class SomeObject2 {
        public $obj2property1;
        public $obj2property2;
}
?>

4. Have that accessible to your script and it will solve the __PHP_Incomplete_Class Object problem as far as the output is concerned. Now you will have:

SomeObject1 ( [obj1property1] => somevalue1 [obj1property2] => SomeObject2 ( [obj2property1] => somevalue1 [obj2property2] => Array ( ['key1'] => somevalue3, ['key2'] => somevalue4 ) ) )

As you will notice, __PHP_Incomplete_Class Object is gone and replaced by the class name. The property __PHP_Incomplete_Class_Name is also removed.

5. As for the array property obj2property2, we can directly access that and just assume that it is an array and loop through it:

<?php

// this will be SomeObject1 
$data = unserialize($serialized_data);

// this will be SomeObject2
$data2 = $data->obj1property2();

foreach($data2->obj2property2 as $key => $value):
         print $key.' : '. $value .'<br>'; 
endforeach;

?>

Outputs:
key1 : somevalue3
key2 : somevalue4

That's it. You can add more methods on the class declarations for the given properties, provided you keep your original output as basis for the data types.

When you serialize an object of a class from a particular namespace, the namespace is recorded as part of the serialization. If you decide to change this namespace's name, it can be hard to read in old serialized objects. I.e., suppose you had serialized an object of type foo\A, you change the namespace of your project to goo but otherwise leave the class definition of A unchanged. You would like to be able to unserialize the object as goo\A, instead unserialization will only create a partial object. To fix this in the case where you don't have nested objects in your class definition, you can use the following simple rename function:
/**
 * Used to change the namespace of a serialized php object (assumes doesn't
 * have nested subobjects)
 *
 * @param string $class_name new fully qualified name with namespace
 * @param string $object_string serialized object
 *
 * @return string serialized object with new name
 */
function renameSerializedObject($class_name, $object_string)
{
    /*  number of digits in the length of name of the object needs to be 
        less than 12 digits (probably more like 4) for this to work.
    */
    $name_length = intval(substr($object_string, 2, 14));
    $name_space_info_length = strlen("O:".$name_length.":") +
        $name_length + 2; // 2 for quotes;
    $object_string = 'O:' .
        strlen($class_name) . ':"'. $class_name.'"' .
        substr($object_string, $name_space_info_length);
    return $object_string;
}

As mentioned in the notes, unserialize returns false in the event of an error and for boolean false. Here is the first solution mentioned, without using error handling:

<?php
function isSerialized($str) {
    return ($str == serialize(false) || @unserialize($str) !== false);
}

var_dump(isSerialized('s:6:"foobar";')); // bool(true)
var_dump(isSerialized('foobar'));        // bool(false)
var_dump(isSerialized('b:0;'));          // bool(true)
?>

When trying to serialize or unserialize recursive arrays or otherwise linked data you might find the undocumented R data type quite useful.

If you want a array like the one produced with
<?
$a = array();
$a[0] =& $a;
?>
serialized you can store it using a string simular to this one:
<?
$a = unserialize("a:1:{i:0;R:1;}");
?>

Both sources will make $a hold an array that self-references itself in index 0.

The argument for R is the index of the created sub-variable of the serialize-string beginning with 1.

When dealing with a string which contain "\r", it seems that the length is not evaluated correctly. The following solves the problem for me :

<?php
// remove the \r caracters from the $unserialized string
$unserialized = str_replace("\r","",$unserialized);

// and then unserialize()
unserialize($unserialized);
?>

Anyone having trouble serializing data with SimpleXMLElement objects stored within it, check this out:

This will traverse $data looking for any children which are instances of SimpleXMLElement, and will run ->asXML() on them, turning them into a string and making them serializable. Other data will be left alone.

<?php
function exportNestedSimpleXML($data) {
    if (is_scalar($data) === false) {
        foreach ($data as $k => $v) {
            if ($v instanceof SimpleXMLElement) {
                $v = str_replace("&#13;","\r",$v->asXML());
            } else {
                $v = exportNestedSimpleXML($v);
            }

            if (is_array($data)) {
                $data[$k] = $v;
            } else if (is_object($data)) {
                $data->$k = $v;
            }
        }
    }

    return $data;
}

$data = array (
    "baz" => array (
        "foo" => new stdClass(),
        "int" => 123,
        "str" => "asdf",
        "bar" => new SimpleXMLElement('<?xml version="1.0" encoding="UTF-8"?><foo>bar</foo>'),
    )
);

var_dump($data);
/*array(1) {
  ["baz"]=>
  array(4) {
    ["foo"]=>
    object(stdClass)#3 (0) {
    }
    ["int"]=>
    int(123)
    ["str"]=>
    string(4) "asdf"
    ["bar"]=>
    object(SimpleXMLElement)#4 (1) {
      [0]=>
      string(3) "bar"
    }
  }
}*/

var_dump(exportNestedSimpleXML($data));
/*array(1) {
  ["baz"]=>
  array(4) {
    ["foo"]=>
    object(stdClass)#3 (0) {
    }
    ["int"]=>
    int(123)
    ["str"]=>
    string(4) "asdf"
    ["bar"]=>
    string(54) "<?xml version="1.0" encoding="UTF-8"?>
<foo>bar</foo>
"
  }
}
*/
?>

Be aware that if useing serialize/unserialize in a serverfarm with both 32bit and 64bit servers you can get unexpected results.

Ex: if you serialize an integer with value of 2147483648 on a 64bit system and then unserialize it on a 32bit system you will get the value -2147483648 instead. This is because an integer on 32bit cannot be above 2147483647 so it wraps.

In reply to the earlier post about having to include object definitions *before* using unserialize.  There is a workaround for this.

When an object is serialized, the first bit of the string is actually the name of the class.  When an unknown object is unserialized, this is maintained as a property.  So if you serialize it again, you get back the exact same string as if you'd serialized the original object.  Basically, to cut to the point...

If you use

$_SESSION['my_object'] = unserialize(serialize($_SESSION['my_object']))

then you get back an object of the correct type, even if the session had originally loaded it as an object of type stdClass.

You can use the following code to use the php 7 unserialize function in php 5.3 and upwards. This adds the $option argument.

<?php

namespace
{

/**
 * PHP 7 unserialize function for PHP 5.3 upwards.
 * Added the $option argument (allowed_classes).
 * See php unserialize manual for more detail.
 **/
function php7_unserialize($str, $options = array())
{
  if(version_compare(PHP_VERSION, '7.0.0', '>='))
  { return unserialize($str, $options); }

  $allowed_classes = isset($options['allowed_classes']) ? 
    $options['allowed_classes'] : true;
  if(is_array($allowed_classes) || !$allowed_classes)
  {
    $str = preg_replace_callback(
      '/(?=^|:)(O|C):\d+:"([^"]*)":(\d+):{/', 
      function($matches) use ($allowed_classes)
      {
        if(is_array($allowed_classes) && 
          in_array($matches[2], $allowed_classes))
        { return $matches[0]; }
        else
        {
          return $matches[1].':22:"__PHP_Incomplete_Class":'.
            ($matches[3] + 1).
            ':{s:27:"__PHP_Incomplete_Class_Name";'.
            serialize($matches[2]);
        }
      },
      $str
    );
  }
  unset($allowed_classes);
  return unserialize($str);
}

} // namespace

namespace my_name_space
{
  /**
   * Use the new php7 unserialize in your namespace without
   * renaming all unserialize(...) function calls to 
   * php7_unserialize(...).
   **/
  function unserialize($str, $options = array())
  { return php7_unserialize($str, $options); }
}

?>

Please note there is a minor difference in how unserializa() works between PHP 7 and 8.

Under PHP 8, strings that are not trimmed of whitespace will emit a warning in the likes of:

PHP Warning:  unserialize(): Extra data starting at offset 721 of 722 bytes in /tmp/a.php on line 4

so something like:

$s = 's:3:"bar";'."\n"
unserialize($s);    # warning emitted
unserialize(trim($s));  # no warning

I was getting unserialize()  Error at offset error.

If you face similar problem  then use the following procedure

$auctionDetails = preg_replace('!s:(\d+):"(.*?)";!se', "'s:'.strlen('$2').':\"$2\";'", $dataArr[$i]['auction_details'] ); 
$auctionDetails = unserialize($auctionDetails);

This little function will check whether the serialized string is well formed. 

PHP < 6 because i'd heard changes will be made in this php-intern function, 
maybe it could be edited easy for it.

<?php

function wd_check_serialization( $string, &$errmsg ) 
{

    $str = 's';
    $array = 'a';
    $integer = 'i';
    $any = '[^}]*?';
    $count = '\d+';
    $content = '"(?:\\\";|.)*?";';
    $open_tag = '\{';
    $close_tag = '\}';
    $parameter = "($str|$array|$integer|$any):($count)" . "(?:[:]($open_tag|$content)|[;])";            
    $preg = "/$parameter|($close_tag)/";
    if( !preg_match_all( $preg, $string, $matches ) ) 
    {            
        $errmsg = 'not a serialized string';
        return false;
    }    
    $open_arrays = 0;
    foreach( $matches[1] AS $key => $value )
    {
        if( !empty( $value ) && ( $value != $array xor $value != $str xor $value != $integer ) ) 
        {
            $errmsg = 'undefined datatype';
            return false;
        }
        if( $value == $array )
        {
            $open_arrays++;                                
            if( $matches[3][$key] != '{' ) 
            {
                $errmsg = 'open tag expected';
                return false;
            }
        }
        if( $value == '' )
        {
            if( $matches[4][$key] != '}' ) 
            {
                $errmsg = 'close tag expected';
                return false;
            }
            $open_arrays--;
        }
        if( $value == $str )
        {
            $aVar = ltrim( $matches[3][$key], '"' );
            $aVar = rtrim( $aVar, '";' );
            if( strlen( $aVar ) != $matches[2][$key] ) 
            {
                $errmsg = 'stringlen for string not match';
                return false;
            }
        }
        if( $value == $integer )
        {
            if( !empty( $matches[3][$key] ) ) 
            {
                $errmsg = 'unexpected data';
                return false;
            }
            if( !is_integer( (int)$matches[2][$key] ) ) 
            {
                $errmsg = 'integer expected';
                return false;
            }
        }
    }        
    if( $open_arrays != 0 ) 
    {
        $errmsg = 'wrong setted arrays';
        return false;
    }
    return true;
}

?>

Beware: file_put_contents() is racy by design and if you use it to write serialize()d data, you may end up having error such as

    unserialize(): Extra data starting at offset 1000 of 1002 bytes

For details, see https://github.com/php/php-src/issues/20108

A quick note:
If you store a serialized object in a session, you have to include the class _before_ you initialize (session_start()) the session.

If serialize() is the answer, you're almost certainly asking the wrong question.

JSON is widely available. The only thing it does not do, is the very thing that makes serialization immensely dangerous. All it takes is a crafty hacker to pass a crafted payload to a supposedly 'secured' serialize call, for a database driver to be overwritten with malicious code, for example.

Recreate the object. Normally. With actual data, and a source file, not with serialize. To do otherwise is laziness bordering on malice.

To check if a string is serialized:

$blSerialized=(@unserialize($sText)||$sText=='b:0;');

For those who are looking for an efficient solution for handling controversial "FALSE", they can use this function which in case of non-unserializable string, instead of a "FALSE", throws an Exception. Vice versa it returns the unserialized variable.
<?php
    /**
     * @param string $serializedString
     * @param array $options
     * @return mixed
     * @throws Exception
     */
    function UnSerialize(string $serializedString, array $options = []) {
        $_unserialized = @unserialize($serializedString, $options);
        if ($serializedString === serialize(false) || $_unserialized !== false){
            return $_unserialized;
        }
        throw new Exception("Non-unserializable string");

    }

?>