How to show a table depending on a drop down selection?

Question

Initially there is only drop down menu in the form, when a selection is made I want to display a table in the same form.

$form['blood_group'] = array(
    '#type' => 'select',
    '#title' => t('Blood Group'), 
    '#options' => array(
        'a+' => 'a +ve',
        'b+' => 'b +ve',
        'ab+' => 'ab +ve',
        'o+' => 'o +ve',
        ), 
    '#required' => TRUE,
);

suppose if the user selects a+ i have to show the table containing names of all the people with the same blood group.

Answer 1

You can take a look at the #ajax property of the Drupal Form API.

Here is a good start :

$form['blood_group_wrapper']['tables'] = array(
  '#prefix' => '<div id="table-wrapper">',
  '#suffix' => '</div>',
);

$form['blood_group_wrapper']['blood_group'] = array(
  '#type' => 'select',
  '#title' => t('Blood Group'), 
  '#options' => array(
      'a+' => 'a +ve',
      'b+' => 'b +ve',
      'ab+' => 'ab +ve',
      'o+' => 'o +ve',
      ), 
  '#required' => TRUE,
  '#ajax' => array(
    'callback' => 'ajax_show_table',
    'wrapper' => 'table-wrapper',
    'method' => 'replace',
    'effect' => 'fade',
  ),
);

function ajax_show_table($form, $form_state) {
  $table = '<table></table>';
  $form['blood_group_wrapper']['tables']['#markup'] = $table;

  return $form['blood_group_wrapper']['tables'];

}