PHP: __toString() and json_encode() not playing well together

In PHP > v5.4.0 you can implement the interface called JsonSerializable as described in the answer by Tivie.

For those of us using PHP < 5.4.0 you can use a solution which employs get_object_vars() from within the object itself and then feeds those to json_encode(). That is what I have done in the following example, using the __toString() method, so that when I cast the object as a string, I get a JSON encoded representation.

Also included is an implementation of the IteratorAggregate interface, with its getIterator() method, so that we can iterate over the object properties as if they were an array.

<?php
class TestObject implements IteratorAggregate {
    
  public $public = "foo";
  protected $protected = "bar";
  private $private = 1;
  private $privateList = array("foo", "bar", "baz" => TRUE);
  
  /**
   * Retrieve the object as a JSON serialized string
   *
   * @return string
   */
  public function __toString() {
    $properties = $this->getAllProperties();

    $json = json_encode(
      $properties,
      JSON_FORCE_OBJECT | JSON_HEX_TAG | JSON_HEX_AMP | JSON_HEX_APOS | JSON_HEX_QUOT
    );

    return $json;
  }

  /**
   * Retrieve an external iterator
   *
   * @link http://php.net/manual/en/iteratoraggregate.getiterator.php
   * @return \Traversable
   *  An instance of an object implementing \Traversable
   */
  public function getIterator() {
    $properties = $this->getAllProperties();
    $iterator = new \ArrayIterator($properties);

    return $iterator;
  }

  /**
   * Get all the properties of the object
   *
   * @return array
   */
  private function getAllProperties() {
    $all_properties = get_object_vars($this);

    $properties = array();
    while (list ($full_name, $value) = each($all_properties)) {
      $full_name_components = explode("\0", $full_name);
      $property_name = array_pop($full_name_components);
      if ($property_name && isset($value)) $properties[$property_name] = $value;
    }

    return $properties;
  }

}

$o = new TestObject();

print "JSON STRING". PHP_EOL;
print "------" . PHP_EOL;
print strval($o) . PHP_EOL;
print PHP_EOL;

print "ITERATE PROPERTIES" . PHP_EOL;
print "-------" . PHP_EOL;
foreach ($o as $key => $val) print "$key -> $val" . PHP_EOL;
print PHP_EOL;

?>

This code produces the following output:

JSON STRING
------
{"public":"foo","protected":"bar","private":1,"privateList":{"0":"foo","1":"bar","baz":true}}

ITERATE PROPERTIES
-------
public -> foo
protected -> bar
private -> 1
privateList -> Array

Isn't your answer in the PHP docs for json_encode?

For anyone who has run into the problem of private properties not being added, you can simply implement the IteratorAggregate interface with the getIterator() method. Add the properties you want to be included in the output into an array in the getIterator() method and return it.

A late answers but might be useful for others with the same problem.

In PHP < 5.4.0 json_encode doesn't call any method from the object. That is valid for getIterator, __serialize, etc...

In PHP > v5.4.0, however, a new interface was introduced, called JsonSerializable.

It basically controls the behaviour of the object when json_encode is called on that object.

---

Example:

class A implements JsonSerializable
{
    protected $a = array();

    public function __construct()
    {
        $this->a = array( new B, new B );
    }

    public function jsonSerialize()
    {
        return $this->a;
    }
}

class B implements JsonSerializable
{
    protected $b = array( 'foo' => 'bar' );

    public function jsonSerialize()
    {
        return $this->b;
    }
}


$foo = new A();

$json = json_encode($foo);

var_dump($json);

---

Outputs:

string(29) "[{"foo":"bar"},{"foo":"bar"}]"