EDIT

      'projection' => NULL,

      'proj4js_def' => NULL,

      'max_extent' => NULL,

    ),

    'show_layer_switcher' => TRUE,

    'display_outside_max_extent' => FALSE,

  $form_elements['name'] = array(

    '#type' => 'textfield',

    '#title' => 'Layer name',

    '#default_value' => $default_settings['name'],

    '#description' => 'A arbitrary name for the layer.',

  );

  $form_elements['url'] = array(

    '#type' => 'textfield',

    '#title' => 'WMS url',

    '#default_value' => $default_settings['url'],

    '#description' => 'Base url for the WMS (e.g.  http://edit.africamuseum.be/geoserver/topp/wms, http://wms.jpl.nasa.gov/wms.cgi)'

  );

  $form_elements['params'] = array(

    '#type' => 'textarea',

    '#title' => 'WMS parameters',

    '#element_validate' => array('form_element_validate_json'),

    '#default_value' => $default_settings['params'],

    '#description' => 'An javasript object with key/value pairs representing the GetMap query string parameters and parameter values ('

      .l('Geoserver WMS parameter reference', 'http://docs.geoserver.org/stable/en/user/services/wms/reference.html#getmap' )

    $form_elements['projection'] = array(

      '#type' => 'textfield',

      '#title' => 'Projection',

      '#default_value' => $default_settings['projection'],

      '#description' => 'The desired projection for the layer (e.g. EPSG:4326, EPSG:900913, EPSG:3857)'

    );

      '#type' => 'textfield',

      '#maxlength' => 256,

      '#title' => 'proj4js definition',

      '#default_value' => $default_settings['proj4js_def'],

      '#description' => 'The <a href="http://trac.osgeo.org/openlayers/wiki/Documentation/Dev/proj4js">proj4js definition</a> for the projection named above.

              The definitions for

    $form_elements['max_extent'] = array(

      '#type' => 'textfield',

      '#title' => 'Maximum extent',

      '#default_value' => $default_settings['max_extent'],

      '#description' => 'The maximum extent of the map as bounding box (left, bottom, right, top) in the units of the map.'

    );