Proposed WP_Object_Classifier for WordPress #metadata.

class-object-classifier.php

<?php

/**
 * Class WP_Object_Classifier
 */
final class WP_Object_Classifier {
  /**
   * @var bool
   */
  var $object_type = false;

  /**
   * @var bool
   */
  var $subtype = false;

  /**
   * @var array
   */
  protected static $_core_object_types = array(
    'post'     => array( 'has_subtype' => true ),
    'user'     => array( 'has_subtype' => false ),
    'comment'  => array( 'has_subtype' => false ),
  );

  /**
   * @param bool|string|array|object $object_type
   */
  function __construct( $object_type = false ) {
    if ( $object_type ) {
      $this->assign( $object_type );
    }
  }

  /**
   * Validated and assigns a value to this Object Classifier
   *
   * @example:
   *
   *     $this->assign(
   *
   * @param bool|string|array|WP_Object_Classifier $object_type
   */
  function assign( $object_type = false ) {

    if ( empty( $object_type ) ) {
      global $post;
      $object_type =  isset( $post->post_type ) ? "post/{$post->post_type}" : 'post/post';
    }

    if ( is_a( $object_type, __CLASS__ ) ) {
      $this->object_type = $object_type->object_type;
      $this->subtype = $object_type->subtype;
    } else {
      if ( is_string( $object_type ) ) {
        if ( isset( self::$_core_object_types[$object_type] ) && ! self::$_core_object_types[$object_type]['has_subtype'] ) {
          $this->object_type = $object_type;
          $this->subtype = false;
        } else {
          if ( false === strpos( $object_type, ':' )  ) {
            $this->object_type = 'post';
            $this->subtype = $object_type;
          } else {
            list( $this->object_type, $this->subtype ) = explode( ':', $object_type );
          }
        }
      } else {
        if ( is_array( $object_type ) ) {
          $object_type = (object)$object_type;
        }
        $this->object_type = property_exists( $object_type, 'object_type' ) ? $object_type->object_type : false;
        $this->subtype = property_exists( $object_type, 'subtype' ) ? $object_type->subtype : false;
      }
      $this->object_type = sanitize_key( $this->object_type );
      if ( $this->subtype ) {
        $this->subtype = sanitize_key( $this->subtype );
      }
    }
  }

  /**
   * Get the most specific type available
   *
   * @return string
   */
  function unqualified_type() {
    return empty( $this->subtype ) ? $this->object_type : $this->subtype;
  }

  /**
   * Check if the current object classifier is valid.
   *
   * @return bool
   */
  public function is_valid() {
    return ! empty( $this->type );
  }

  /**
   * Check if the current object classifier is equivalent to the one passed in.
   *
   * @param WP_Object_Classifier $that
   * @return bool
   */
  public function is_equivalent( $that ) {
    if ( ! is_a( $that, __CLASS__ ) ) {
      $object_type = new self( $that );
    }
    return $this == $that;
  }

  /**
   * @return string
   */
  function __toString() {
   return "{$this->object_type}/{$this->subtype}";
  }

}