Type classes need to extend the abstract DataType class. Besides providing a means of categorizing types, the DataType class provides the methods necessary to support the "refid" attribute. (All types can be given an id, and can be referred to later using that id.)

In this example we are creating a DSN type because we have written a number of DB-related Tasks, each of which need to know how to connect to the database; instead of having database parameters for each task, we've created a DSN type so that we can identify the connection parameters once and then use it in all our db Tasks.

require_once "phing/types/DataType.php";

/**
 * This Type represents a DB Connection.
 */
class DSN extends DataType {

  private $url;
  private $username;
  private $password;
  private $persistent = false;

  /**
   * Sets the URL part: mysql://localhost/mydatabase
   */
  public function setUrl($url) {
    $this->url = $url;
  }

  /**
   * Sets username to use in connection.
   */
  public function setUsername($username) {
    $this->username = $username;
  }

  /**
   * Sets password to use in connection.
   */
  public function setPassword($password) {
    $this->password = $password;
  }

  /**
   * Set whether to use persistent connection.
   * @param boolean $persist
   */
  public function setPersistent($persist) {
    $this->persistent = (boolean) $persist;
  }

  public function getUrl(Project $p) {
    if ($this->isReference()) {
      return $this->getRef($p)->getUrl($p);
    }
    return $this->url;
  }

  public function getUsername(Project $p) {
    if ($this->isReference()) {
      return $this->getRef($p)->getUsername($p);
    }
    return $this->username;
  }

  public function getPassword(Project $p) {
    if ($this->isReference()) {
      return $this->getRef($p)->getPassword($p);
    }
    return $this->password;
  }

  public function getPersistent(Project $p) {
    if ($this->isReference()) {
      return $this->getRef($p)->getPersistent($p);
    }
    return $this->persistent;
  }

  /**
   * Gets a combined hash/array for DSN as used by PEAR.
   * @return array
   */
  public function getPEARDSN(Project $p) {
    if ($this->isReference()) {
      return $this->getRef($p)->getPEARDSN($p);
    }

    include_once 'DB.php';
    $dsninfo = DB::parseDSN($this->url);
    $dsninfo['username'] = $this->username;
    $dsninfo['password'] = $this->password;
    $dsninfo['persistent'] = $this->persistent;

    return $dsninfo;
  }

  /**
   * Your datatype must implement this function, which ensures that there
   * are no circular references and that the reference is of the correct
   * type (DSN in this example).
   *
   * @return DSN
   */
  public function getRef(Project $p) {
    if ( !$this->checked ) {
      $stk = array();
      array_push($stk, $this);
      $this->dieOnCircularReference($stk, $p);
    }
    $o = $this->ref->getReferencedObject($p);
    if ( !($o instanceof DSN) ) {
      throw new BuildException($this->ref->getRefId()." doesn't denote a DSN");
    } else {
      return $o;
    }
  }

}

The TypedefTask provides a way to "declare" your type so that you can use it in your build file. Here is how you would use this type in order to define a single DSN and use it for multiple tasks. (Of course you could specify the DSN connection parameters each time, but the premise behind needing a DSN datatype was to avoid specifying the connection parameters for each task.)

<?xml version="1.0" ?>

<project name="test" basedir=".">

  <typedef name="dsn" classname="myapp.types.DSN" />

  <dsn
      id="maindsn"
      url="mysql://localhost/mydatabase"
      username="root"
      password=""
      persistent="false" />

  <target name="main">

    <my-special-db-task>
         <dsn refid="maindsn"/>
    </my-special-db-task>

    <my-other-db-task>
      <dsn refid="maindsn"/>
    </my-other-db-task>

  </target>

</project>