class User {
  /**
   * ...some mapping info...
   */
  private $username;

  /**
   * ...some mapping info...
   */
  private $password;

  public function setUsername($username) {
    $this->username = $username;
  }

  public function setPassword($password) {
    $this->password = $password;
  }
}

<form action="/controller/saveUser" method="post">
  <p>Username: <input type="text" name="username" /></p>
  <p>Password: <input type="text" name="password" /></p>
</form>

public function saveUser() {
  $user = new User();
  $user->setUsername($_POST['username']);
  $user->setPassword($_POST['password']);

  $entityManager->persist($user);
}

public function saveUser() {
  $user = Doctrine::populateModelFromArray('User', $_POST); // does this method exist?
  $entityManager->persist($user);
}

<?php
foreach($_POST as $field => $val){
  $object->$field = $val;
}
?>