source: sipei/modules/genpass/genpass.module @ e7955cb

<?php

function _genpass_default_entropy() {
  return 'abcdefghjkmnpqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ23456789!#$%&()*+,-./:;<=>?@[]^_{|}~';
}

/**
 * Generate a new password
 * Based on the original D6 user_password function (with more characters)
 *
 * @return a fresh password according to the settings made in /admin/user/settings
 *
 * @see genpass_form_alter()
 */
function genpass_password() {
  $pass = '';
  $length = variable_get('genpass_length', 8);
  $allowable_characters = variable_get('genpass_entropy', _genpass_default_entropy());

  // Zero-based count of characters in the allowable list:
  $len = strlen($allowable_characters) - 1;
  
  // Loop the number of times specified by $length.
  for ($i = 0; $i < $length; $i++) {
    // Each iteration, pick a random character from the
    // allowable string and append it to the password:
    $pass .= $allowable_characters[mt_rand(0, $len)];
  }

  return $pass;
}


/**
 * helper function to find the password item in a form, since its position within
 * the form-array depends on the profile module (account-category)
 */ 
function &genpass_get_pass_item(&$form){
  if(isset($form['account']['pass'])) return $form['account']['pass'];
  else return $form['pass'];
}




/**
 * Take care of new form handling (admin and registration forms)
 */
function genpass_form_alter(&$form, $form_state, $form_id) {
  switch($form_id) {
  
    case 'user_admin_settings':

      $form['registration']['genpass_mode'] = array(
        '#type' => 'radios',
        '#title' => t('Password handling'),
        '#default_value' => variable_get('genpass_mode', 0),
        '#options' => array(
          0 => t('Users must enter a password on registration (default Drupal behaviour).'),
          1 => t('Users may enter a password on registration. If not, a random password will be generated.'),
          2 => t('Users cannot choose a password on registration. It will be generated.')
        ),
        '#description' => t('Choose a password handling mode for new users. Note that for manual user creation by an administrator the second option always applies.')
      );
      $form['registration']['genpass_length'] = array(
        '#type' => 'textfield',
        '#title' => t('Generated password length'),
        '#default_value' => variable_get('genpass_length', 8),
        '#size' => 2,
        '#maxlength' => 2,
        '#description' => t('Set the length of generated passwords here. Allowed range: 5 to 32')
      );
      $form['registration']['genpass_entropy'] = array(
        '#type' => 'textfield',
        '#title' => t('Generated password entropy'),
        '#default_value' => variable_get('genpass_entropy', _genpass_default_entropy()),
        '#description' => t('Give a list of possible characters for a generated password. Note that the list must contain at least X different characters where X is defined by the length you have given above.')
      ); 
      $form['#validate'][] = 'genpass_user_admin_settings_validate';
      $form['#submit'][] = 'genpass_user_admin_settings_submit';
      break;
      
    case 'user_register':

      $pass_item =& genpass_get_pass_item($form);
      // alter the user-admin register (no mode decision)
      if ($form['destination']['#value'] == 'admin/user/user/create' && user_access('administer users')) { 
        $pass_item['#required'] = FALSE;
        $form['#validate'][] = 'genpass_user_admin_register_validate';
      } 
      else { // alter the user-register form according to the mode
        switch(variable_get('genpass_mode',0)){
          case 2: 
            $pass_item['#type'] = 'hidden';
          case 1: 
            $pass_item['#required'] = FALSE;
            $form['#validate'][] = 'genpass_register_validate';
        }
      }

      break;
  }
  
}


function genpass_user_admin_settings_validate($form, &$form_state) {
  $length = $form_state['values']['genpass_length'];
  if(!is_numeric($length) || $length < 5 || $length > 32){
    form_set_error('genpass_length', t('The length of a generated password must be between 5 and 32')); return;
  }
  $entropy = $form_state['values']['genpass_entropy'];
  $chars = array_unique(preg_split('//', $entropy, -1, PREG_SPLIT_NO_EMPTY));
  if(count($chars) < $length){ 
    form_set_error('genpass_entropy', t('The list of possible characters is not long or unique enough')); return;
  }
  return $form;
}


function genpass_user_admin_settings_submit($form, &$form_state) {
  $mode = $form_state['values']['genpass_mode'];
  if($mode >= 0 && $mode < 3) variable_set('genpass_mode', $mode);
  
  $length = $form_state['values']['genpass_length'];
  if(is_numeric($length) && $length >= 5 && $length <= 32) variable_set('genpass_length', $length);
  
  $entropy = $form_state['values']['genpass_entropy'];
  $chars = array_unique(preg_split('//', $entropy, -1, PREG_SPLIT_NO_EMPTY));
  if(count($chars) >= $length) variable_set('genpass_entropy',$entropy);
}



function genpass_register_validate($form, &$form_state) {
  if (empty($form_state['values']['pass']) && !form_get_errors() ) {
    $pass = genpass_password();
    $pass_item =& genpass_get_pass_item($form);
    form_set_value($pass_item, $pass, $form_state);
    drupal_set_message(t('Generated password'));
  }
  return $form;
}


function genpass_user_admin_register_validate($form, &$form_state) {
  if (empty($form_state['values']['pass']) && !form_get_errors() ) {
    $pass = genpass_password();
    $pass_item =& genpass_get_pass_item($form);
    form_set_value($pass_item, $pass, $form_state);
    drupal_set_message(t('Generated password is ').$pass);
  }
  return $form;
}