Driver Architecture and Internals
PHP Manual

Serialization and deserialization of PHP variables into MongoDB

This document discusses how compound structures (i.e. documents, arrays, and objects) are converted between BSON and PHP values.

Serialization to BSON

Arrays

If an array is a packed array — i.e. empty array or the keys start at 0 and are sequential without gaps: BSON array.

If the array is not packed — i.e. having associative (string) keys, the keys don't start at 0, or when there are gaps:: BSON object

A top-level (root) document, always serializes as a BSON document.

Examples

These serialize as a BSON array: 

[ 8, 5, 2, 3 ] => [ 8, 5, 2, 3 ]
[ 0 => 4, 1 => 9 ] => [ 4, 9 ]

These serialize as a BSON document: 

[ 0 => 1, 2 => 8, 3 => 12 ] => { "0" : 1, "2" : 8, "3" : 12 }
[ "foo" => 42 ] => { "foo" : 42 }
[ 1 => 9, 0 => 10 ] => { "1" : 9, "0" : 10 }

Note that the five examples are extracts of a full document, and represent only one value inside a document.

Objects

If an object is of the stdClass class, serialize as a BSON document.

If an object is a supported class that implements MongoDB\BSON\Type, then use the BSON serialization logic for that specific type. MongoDB\BSON\Type instances (excluding MongoDB\BSON\Serializable may only be serialized as a document field value. Attempting to serialize such an object as a root document will throw a MongoDB\Driver\Exception\UnexpectedValueException

If an object is of an unknown class implementing the MongoDB\BSON\Type interface, then throw a MongoDB\Driver\Exception\UnexpectedValueException

If an object is of any other class, without implementing any special interface, serialize as a BSON document. Keep only public properties, and ignore protected and private properties.

If an object is of a class that implements the MongoDB\BSON\Serializable interface, call MongoDB\BSON\Serializable::bsonSerialize and use the returned array or stdClass to serialize as a BSON document or array. The BSON type will be determined by the following:

  1. Root documents must be serialized as a BSON document.

  2. MongoDB\BSON\Persistable objects must be serialized as a BSON document.

  3. If MongoDB\BSON\Serializable::bsonSerialize returns a packed array, serialize as a BSON array.

  4. If MongoDB\BSON\Serializable::bsonSerialize returns a non-packed array or stdClass, serialize as a BSON document.

  5. If MongoDB\BSON\Serializable::bsonSerialize did not return an array or stdClass, throw an MongoDB\Driver\Exception\UnexpectedValueException exception.

If an object is of a class that implements the MongoDB\BSON\Persistable interface (which implies MongoDB\BSON\Serializable), obtain the properties in a similar way as in the previous paragraphs, but also add an additional property __pclass as a Binary value, with subtype 0x80 and data bearing the fully qualified class name of the object that is being serialized.

The __pclass property is added to the array or object returned by MongoDB\BSON\Serializable::bsonSerialize, which means it will overwrite any __pclass key/property in the MongoDB\BSON\Serializable::bsonSerialize return value. If you want to avoid this behaviour and set your own __pclass value, you must not implement MongoDB\BSON\Persistable and should instead implement MongoDB\BSON\Serializable directly.

Examples

<?php

class stdClass
{
    public $foo = 42;
} // => {"foo": 42}

class MyClass
{
    public $foo = 42;
    protected $prot = 'wine';
    private $fpr = 'cheese';
} // => {"foo": 42}

class AnotherClass1 implements MongoDB\BSON\Serializable
{
    public $foo = 42;
    protected $prot = 'wine';
    private $fpr = 'cheese';

    public function bsonSerialize(): array
    {
        return ['foo' => $this->foo, 'prot' => $this->prot];
    }
} // => {"foo": 42, "prot": "wine"}

class AnotherClass2 implements MongoDB\BSON\Serializable
{
    public $foo = 42;

    public function bsonSerialize(): self
    {
        return $this;
    }
} // => MongoDB\Driver\Exception\UnexpectedValueException("bsonSerialize() did not return an array or stdClass")

class AnotherClass3 implements MongoDB\BSON\Serializable
{
    private $elements = ['foo', 'bar'];

    public function bsonSerialize(): array
    {
        return $this->elements;
    }
} // => {"0": "foo", "1": "bar"}

/**
 * Nesting Serializable classes
 */

class AnotherClass4 implements MongoDB\BSON\Serializable
{
    private $elements = [0 => 'foo', 2 => 'bar'];

    public function bsonSerialize(): array
    {
        return $this->elements;
    }
} // => {"0": "foo", "2": "bar"}

class ContainerClass1 implements MongoDB\BSON\Serializable
{
    public $things;

    public function __construct()
    {
        $this->things = new AnotherClass4();
    }

    function bsonSerialize(): array
    {
        return ['things' => $this->things];
    }
} // => {"things": {"0": "foo", "2": "bar"}}


class AnotherClass5 implements MongoDB\BSON\Serializable
{
    private $elements = [0 => 'foo', 2 => 'bar'];

    public function bsonSerialize(): array
    {
        return array_values($this->elements);
    }
} // => {"0": "foo", "1": "bar"} as a root class
        ["foo", "bar"] as a nested value

class ContainerClass2 implements MongoDB\BSON\Serializable
{
    public $things;

    public function __construct()
    {
        $this->things = new AnotherClass5();
    }

    public function bsonSerialize(): array
    {
        return ['things' => $this->things];
    }
} // => {"things": ["foo", "bar"]}


class AnotherClass6 implements MongoDB\BSON\Serializable
{
    private $elements = ['foo', 'bar'];

    function bsonSerialize(): object
    {
        return (object) $this->elements;
    }
} // => {"0": "foo", "1": "bar"}

class ContainerClass3 implements MongoDB\BSON\Serializable
{
    public $things;

    public function __construct()
    {
        $this->things = new AnotherClass6();
    }

    public function bsonSerialize(): array
    {
        return ['things' => $this->things];
    }
} // => {"things": {"0": "foo", "1": "bar"}}

class UpperClass implements MongoDB\BSON\Persistable
{
    public $foo = 42;
    protected $prot = 'wine';
    private $fpr = 'cheese';

    private $data;

    public function bsonUnserialize(array $data): void
    {
        $this->data = $data;
    }

    public function bsonSerialize(): array
    {
        return ['foo' => $this->foo, 'prot' => $this->prot];
    }
} // => {"foo": 42, "prot": "wine", "__pclass": {"$type": "80", "$binary": "VXBwZXJDbGFzcw=="}}

?>

Deserialization from BSON

Warning

BSON documents can technically contain duplicate keys because documents are stored as a list of key-value pairs; however, applications should refrain from generating documents with duplicate keys as server and driver behavior may be undefined. Since PHP objects and arrays cannot have duplicate keys, data could also be lost when decoding a BSON document with duplicate keys.

The legacy mongo extension deserialized both BSON documents and arrays as PHP arrays. While PHP arrays are convenient to work with, this behavior was problematic because different BSON types could deserialize to the same PHP value (e.g. {"0": "foo"} and ["foo"]) and make it impossible to infer the original BSON type. By default, the mongodb extension addresses this concern by ensuring that BSON arrays and documents are converted to PHP arrays and objects, respectively.

For compound types, there are three data types:

root

refers to the top-level BSON document only

document

refers to embedded BSON documents only

array

refers to a BSON array

Besides the three collective types, it is also possible to configure specific fields in your document to map to the data types mentioned below. As an example, the following type map allows you to map each embedded document within an "addresses" array to an Address class and each "city" field within those embedded address documents to a City class: 

[
    'fieldPaths' => [
        'addresses.$' => 'MyProject\Address',
        'addresses.$.city' => 'MyProject\City',
    ],
]

Each of those three data types, as well as the field specific mappings, can be mapped against different PHP types. The possible mapping values are:

not set or NULL (default)

  • A BSON array will be deserialized as a PHP array.

  • A BSON document (root or embedded) without a __pclass property [1] becomes a PHP stdClass object, with each BSON document key set as a public stdClass property.

  • A BSON document (root or embedded) with a __pclass property [1] becomes a PHP object of the class name as defined by the __pclass property.

    If the named class implements the MongoDB\BSON\Persistable interface, then the properties of the BSON document, including the __pclass property, are sent as an associative array to the MongoDB\BSON\Unserializable::bsonUnserialize function to initialise the object's properties.

    If the named class does not exist or does not implement the MongoDB\BSON\Persistable interface, stdClass will be used and each BSON document key (including __pclass) will be set as a public stdClass property.

    The __pclass functionality relies on the property being part of a retrieved MongoDB document. If you use a projection when querying for documents, you need to include the __pclass field in the projection for this functionality to work.

"array"

Turns a BSON array or BSON document into a PHP array. There will be no special treatment of a __pclass property [1], but it may be set as an element in the returned array if it was present in the BSON document.

"object" or "stdClass"

Turns a BSON array or BSON document into a stdClass object. There will be no special treatment of a __pclass property [1], but it may be set as a public property in the returned object if it was present in the BSON document.

"bson"

Turns a BSON array into a MongoDB\BSON\PackedArray and a BSON document into a MongoDB\BSON\Document, regardless of whether the BSON document has a __pclass property [1].

Note: The bson value is only available for the three root types, not in the field specific mappings.

any other string

Defines the class name that the BSON array or BSON object should be deserialized as. For BSON objects that include __pclass properties, that class will take priority.

If the named class does not exist, is not concrete (i.e. it is abstract or an interface), or does not implement MongoDB\BSON\Unserializable then an MongoDB\Driver\Exception\InvalidArgumentException exception is thrown.

If the BSON object has a __pclass property and that class exists and implements MongoDB\BSON\Persistable it will supersede the class provided in the type map.

The properties of the BSON document, including the __pclass property if it exists, will be sent as an associative array to the MongoDB\BSON\Unserializable::bsonUnserialize function to initialise the object's properties.

TypeMaps

TypeMaps can be set through the MongoDB\Driver\Cursor::setTypeMap method on a MongoDB\Driver\Cursor object, or the $typeMap argument of MongoDB\BSON\toPHP, MongoDB\BSON\Document::toPHP, and MongoDB\BSON\PackedArray::toPHP. Each of the three classes (root, document, and array) can be individually set, in addition to the field specific types.

If the value in the map is NULL, it means the same as the default value for that item.

Examples

These examples use the following classes:

MyClass

which does not implement any interface

YourClass

which implements MongoDB\BSON\Unserializable

OurClass

which implements MongoDB\BSON\Persistable

TheirClass

which extends OurClass

The MongoDB\BSON\Unserializable::bsonUnserialize method of YourClass, OurClass, TheirClass iterate over the array and set the properties without modifications. It also sets the $unserialized property to true: 

<?php

function bsonUnserialize( array $map )
{
    foreach ( $map as $k => $value )
    {
        $this->$k = $value;
    }
    $this->unserialized = true;
}

/* typemap: [] (all defaults) */
{ "foo": "yes", "bar" : false }
  -> stdClass { $foo => 'yes', $bar => false }

{ "foo": "no", "array" : [ 5, 6 ] }
  -> stdClass { $foo => 'no', $array => [ 5, 6 ] }

{ "foo": "no", "obj" : { "embedded" : 3.14 } }
  -> stdClass { $foo => 'no', $obj => stdClass { $embedded => 3.14 } }

{ "foo": "yes", "__pclass": "MyClass" }
  -> stdClass { $foo => 'yes', $__pclass => 'MyClass' }

{ "foo": "yes", "__pclass": { "$type" : "80", "$binary" : "MyClass" } }
  -> stdClass { $foo => 'yes', $__pclass => Binary(0x80, 'MyClass') }

{ "foo": "yes", "__pclass": { "$type" : "80", "$binary" : "YourClass") }
  -> stdClass { $foo => 'yes', $__pclass => Binary(0x80, 'YourClass') }

{ "foo": "yes", "__pclass": { "$type" : "80", "$binary" : "OurClass") }
  -> OurClass { $foo => 'yes', $__pclass => Binary(0x80, 'OurClass'), $unserialized => true }

{ "foo": "yes", "__pclass": { "$type" : "44", "$binary" : "YourClass") }
  -> stdClass { $foo => 'yes', $__pclass => Binary(0x44, 'YourClass') }

/* typemap: [ "root" => "MissingClass" ] */
{ "foo": "yes" }
  -> MongoDB\Driver\Exception\InvalidArgumentException("MissingClass does not exist")

/* typemap: [ "root" => "MyClass" ] */
{ "foo": "yes", "__pclass" : { "$type": "80", "$binary": "MyClass" } }
  -> MongoDB\Driver\Exception\InvalidArgumentException("MyClass does not implement Unserializable interface")

/* typemap: [ "root" => "MongoDB\BSON\Unserializable" ] */
{ "foo": "yes" }
  -> MongoDB\Driver\Exception\InvalidArgumentException("Unserializable is not a concrete class")

/* typemap: [ "root" => "YourClass" ] */
{ "foo": "yes", "__pclass" : { "$type": "80", "$binary": "MongoDB\BSON\Unserializable" } }
  -> YourClass { $foo => "yes", $__pclass => Binary(0x80, "MongoDB\BSON\Unserializable"), $unserialized => true }

/* typemap: [ "root" => "YourClass" ] */
{ "foo": "yes", "__pclass" : { "$type": "80", "$binary": "MyClass" } }
  -> YourClass { $foo => "yes", $__pclass => Binary(0x80, "MyClass"), $unserialized => true }

/* typemap: [ "root" => "YourClass" ] */
{ "foo": "yes", "__pclass" : { "$type": "80", "$binary": "OurClass" } }
  -> OurClass { $foo => "yes", $__pclass => Binary(0x80, "OurClass"), $unserialized => true }

/* typemap: [ "root" => "YourClass" ] */
{ "foo": "yes", "__pclass" : { "$type": "80", "$binary": "TheirClass" } }
  -> TheirClass { $foo => "yes", $__pclass => Binary(0x80, "TheirClass"), $unserialized => true }

/* typemap: [ "root" => "OurClass" ] */
{ foo: "yes", "__pclass" : { "$type": "80", "$binary": "TheirClass" } }
  -> TheirClass { $foo => "yes", $__pclass => Binary(0x80, "TheirClass"), $unserialized => true }

/* typemap: [ 'root' => 'YourClass' ] */
{ foo: "yes", "__pclass" : { "$type": "80", "$binary": "YourClass" } }
  -> YourClass { $foo => 'yes', $__pclass => Binary(0x80, 'YourClass'), $unserialized => true }

/* typemap: [ 'root' => 'array', 'document' => 'array' ] */
{ "foo": "yes", "bar" : false }
  -> [ "foo" => "yes", "bar" => false ]

{ "foo": "no", "array" : [ 5, 6 ] }
  -> [ "foo" => "no", "array" => [ 5, 6 ] ]

{ "foo": "no", "obj" : { "embedded" : 3.14 } }
  -> [ "foo" => "no", "obj" => [ "embedded => 3.14 ] ]

{ "foo": "yes", "__pclass": "MyClass" }
  -> [ "foo" => "yes", "__pclass" => "MyClass" ]

{ "foo": "yes", "__pclass" : { "$type": "80", "$binary": "MyClass" } }
  -> [ "foo" => "yes", "__pclass" => Binary(0x80, "MyClass") ]

{ "foo": "yes", "__pclass" : { "$type": "80", "$binary": "OurClass" } }
  -> [ "foo" => "yes", "__pclass" => Binary(0x80, "OurClass") ]

/* typemap: [ 'root' => 'object', 'document' => 'object' ] */
{ "foo": "yes", "__pclass": { "$type": "80", "$binary": "MyClass" } }
  -> stdClass { $foo => "yes", "__pclass" => Binary(0x80, "MyClass") }

[1] A __pclass property is only deemed to exist if there exists a property with that name, and it is a Binary value, and the sub-type of the Binary value is 0x80. If any of these three conditions is not met, the __pclass property does not exist and should be treated as any other normal property.