Creating Autocomplete Text Fields

It's simple with form API. You must have set autocomplete path with autocomplete function callback in your hook_menu() .

<?php
  
    // path with autocomplete function for cities
    $items['todo/autocomplete'] = array(
      'path' => 'cities/autocomplete',
      'title' => 'Autocomplete for cities',
      'callback' => '_cities_autocomplete',
      'access' => TRUE,
      'type' => MENU_CALLBACK
    );?>

And in your form:

<?php
  $form['city'] = array(
    '#type' => 'textfield',
    '#title' => 'City',
    '#maxlength' => 128,
    '#autocomplete_path' => 'cities/autocomplete',
  );?>

And autocomplete function which search city in table cities looks like:

<?php/**
* autocomplete helper
* $string = string for search
*/function _cities_autocomplete($string) {
  $matches = array();
  // searching in database, only city column
  $result = db_query_range("SELECT city FROM {cities} WHERE LOWER(city) LIKE LOWER('%s%%')", $string, 0, 10);
  // found wrote into $matches
  while ($data = db_fetch_object($result)) {
    $matches[$data->city] = check_plain($data->city);
  }
  // return for JS
  //print drupal_to_js($matches);
drupal_json($matches);  


// we don't need goto next PHP
  exit();
}?>