<?php
class user_powermailCounter {
  var $cObj;
 
  function main ($content,$conf) {
        $select_fields = 'uid';
        $from_table    = 'tx_powermail_mails';
        $where_clause  = '';
        $groupBy       = '';
        $orderBy       = 'uid DESC';
        $limit         = '0,1';
 
        $result = $GLOBALS['TYPO3_DB']->exec_SELECTquery(
          	    $select_fields,
  		    $from_table,
  		    $where_clause,
  		    $groupBy,
  		    $orderBy,
  		    $limit);  	    
 
        $row = $GLOBALS['TYPO3_DB']->sql_fetch_row($result);
        $GLOBALS['TYPO3_DB']->sql_free_result($result);
 
   return '<input type="hidden" name="tx_powermail_pi1[uid999]" value="'.sprintf("%05d",$row[0]+1).'"/>';
  }
}
?>

includeLibs.powermailCounter = fileadmin/template/phpscripts/powermailCounter.php
 
lib.powermailCounter = USER
lib.powermailCounter {
  userFunc =user_powermailCounter->main
}