demonstrates using a plugin to add a custom form element that simply prints out any HTML

pdb-html-form-element-example.php

<?php

/**
 * Plugin Name: PDB HTML Form Element
 * Description: a general-purpose form element that simply displays HTML
 * 
 */
class PDb_html_form_element_example {

  /**
   * @var string element slug 
   */
  protected $name = 'html';

  /**
   * @var string title of the form element
   */
  protected $title = 'HTML';

  /**
   * instantiates the class
   */
  public function __construct()
  {
    // register the form element
    add_filter( 'pdb-set_form_element_types', array($this, 'register_element') );
    // build the element html in a form context
    add_action( 'pdb-form_element_build_' . $this->name, array($this, 'element_build') );
    // build the form element html in a non-form context
    add_filter( 'pdb-before_display_form_element', array($this, 'element_value_display'), 10, 2 );
    // hide the element from non-enabled shortcodes
    add_filter( 'pdb-before_field_added_to_iterator', array($this, 'element_alter_field_list') );
  }

  /**
   * adds the form element to the list of form elements
   * 
   * @param array $elements
   * @return html
   */
  public function register_element( $elements )
  {
    $elements[$this->name] = $this->title;
    return $elements;
  }

  /**
   * supplies the form element HTML in a form context
   *
   * @param PDb_FormElement $field
   * @return null
   */
  public function element_build( $field )
  {
    $field->output = '<span class="html-element-container">' . $this->get_html( $field ) . '</span>';
  }

  /**
   * defines the HTML to show in a non-form context
   *
   * @param string $value the raw value of the field
   * @param PDb_Field_Item $field
   * @return string HTML
   */
  public function element_value_display( $value, $field )
  {
    // if the current field is our form element, provide the html
    if ( $field->form_element === $this->name ) {
      $value = '<span class="html-element-value">' . $this->get_html( $field ) . '</span>';
    }
    return $value;
  }

  /**
   * removes the field from the list of field to show
   * 
   * this works by changing the field type to "hidden" so it won't be shown in any 
   * module not in the list of enabled modules
   * 
   * @param object $field current field object
   * @return object the field object
   */
  public function element_alter_field_list( $field )
  {
    /*
     * if the current field is our form element and the current module is not one 
     * that has been enabled, hide the element
     */
    if ( $field->form_element === $this->name && !in_array( $field->module, $this->get_enabled_modules( $field ) ) ) {
      $field->form_element = 'hidden';
    }
    return $field;
  }

  /**
   * provides the value in string form
   * 
   * @param object $field the field object
   * @return string
   */
  private function get_html( $field )
  {
    // get the field definition
    $field_def = Participants_Db::$fields[$field->name];
    /*
     * we go through all this because the values property could come in 
     * as an array, a serialized array or a string. By doing this we ensure 
     * the result will in all cases be a string.
     */
    return current( (array) maybe_unserialize( $field_def->values ) );
  }

  /**
   * supplies a list of modules where this form element is enabled
   * 
   * @param object $field the current field object
   * @return array of module names
   */
  protected function get_enabled_modules( $field )
  {
    // determine the setting value ot use a default
    $default = empty( $field->default ) ? 'signup' : $field->default;
    // make the setting into an array of values
    return explode( ',', str_replace( ' ', '', $default ) );
  }

}
// instantiate the class so it will be active
new PDb_html_form_element_example();