Emit callbacks

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- Emit callbacks

## Emit callbacks

Emit callbacks are invoked when an instance of a registered class is emitted by yaml_emit() or yaml_emit_file(). The callback is passed the object to be emitted. The callback must return an array having two keys: "`tag`" and "`data`". The value associated with the "`tag`" key must be a string to be used as the YAML tag in the output. The value associated with the "`data`" key will be encoded as YAML and emitted in place of the intercepted object.

Example #1 Emit callback example

    <?phpclass EmitExample {  public $data;    // data may be in any pecl/yaml suitable type  public function __construct ($d) {    $this->data = $d;  }  /**   * Yaml emit callback function, referred on yaml_emit call by class name.   *   * Expected to return an array with 2 values:   *   - 'tag': custom tag for this serialization   *   - 'data': value to convert to yaml (array, string, bool, number)   *   * @param object $obj Object to be emitted   * @return array Tag and surrogate data to emit   */  public static function yamlEmit (EmitExample $obj) {    return array(      'tag' => '!example/emit',      'data' => $obj->data,    );  }}$emit_callbacks = array(  'EmitExample' => array('EmitExample', 'yamlEmit'));$t = new EmitExample(array('a','b','c'));$yaml = yaml_emit(  array(    'example' => $t,  ),  YAML_ANY_ENCODING,  YAML_ANY_BREAK,  $emit_callbacks);var_dump($yaml);?>

The above example will output something similar to:

    string(43) "---
    example: !example/emit
    - a
    - b
    - c
    ...
    "