# Advanced Usage of Zend_Json

## JSON Objects

When encoding PHP objects as JSON, all public properties of that object will be encoded in a JSON object.

JSON does not allow object references, so care should be taken not to encode objects with recursive references. If you have issues with recursion, Zend_Json::encode() and Zend_Json_Encoder::encode() allow an optional second parameter to check for recursion; if an object is serialized twice, an exception will be thrown.

Decoding JSON objects poses an additional difficulty, however, since Javascript objects correspond most closely to PHP's associative array. Some suggest that a class identifier should be passed, and an object instance of that class should be created and populated with the key/value pairs of the JSON object; others feel this could pose a substantial security risk.

By default, Zend_Json will decode JSON objects as associative arrays. However, if you desire an object returned, you can specify this:

    // Decode JSON objects as PHP objects
    $phpNative = Zend_Json::decode($encodedValue, Zend_Json::TYPE_OBJECT);

Any objects thus decoded are returned as StdClass objects with properties corresponding to the key/value pairs in the JSON notation.

The recommendation of Zend Framework is that the individual developer should decide how to decode JSON objects. If an object of a specified type should be created, it can be created in the developer code and populated with the values decoded using Zend_Json.

## Encoding PHP objects

If you are encoding PHP objects by default the encoding mechanism can only access public properties of these objects. When a method toJson() is implemented on an object to encode, Zend_Json calls this method and expects the object to return a JSON representation of its internal state.

## Internal Encoder/Decoder

Zend_Json has two different modes depending if ext/json is enabled in your PHP installation or not. If ext/json is installed by default json_encode() and json_decode() functions are used for encoding and decoding JSON. If ext/json is not installed a Zend Framework implementation in PHP code is used for en-/decoding. This is considerably slower than using the PHP extension, but behaves exactly the same.

Still sometimes you might want to use the internal encoder/decoder even if you have ext/json installed. You can achieve this by calling:

    Zend_Json::$useBuiltinEncoderDecoder = true:

## JSON Expressions

Javascript makes heavy use of anonymnous function callbacks, which can be saved within JSON object variables. Still they only work if not returned inside double qoutes, which Zend_Json naturally does. With the Expression support for Zend_Json support you can encode JSON objects with valid javascript callbacks. This works for both json_encode() or the internal encoder.

A javascript callback is represented using the Zend_Json_Expr object. It implements the value object pattern and is immutable. You can set the javascript expression as the first constructor argument. By default Zend_Json::encode does not encode javascript callbacks, you have to pass the option enableJsonExprFinder and set it to `TRUE` into the encode() function. If enabled the expression support works for all nested expressions in large object structures. A usage example would look like:

    $data = array(
        'onClick' => new Zend_Json_Expr('function() {'
                  . 'alert("I am a valid javascript callback '
                  . 'created by Zend_Json"); }'),
        'other' => 'no expression',
    );
    $jsonObjectWithExpression = Zend_Json::encode(
        $data,
        false,
        array('enableJsonExprFinder' => true)
    );