trying to print out a user's articles by setting a placeholder

/* 
* This plugin detects the user's user id through the modx api and displays the articles that have been posted by that user.
* Created: 19-10-2007
* Author: Sarah Vardy
*/

$uid = '';
$parentID = 48;
$articleListingForUser = '';

// Set uid to the login user id
$uid = modx->getLoginUserID();

// Query the database to find out what articles have been posted by the user

//select the modx table to use
$table = $modx->getFullTableName("site_content");

//iterate through the table and see what articles have been posted by the user

$result = $modx->db->select('*', $table); 

//while there are still more rows in the table check to see if the user is logged in

while ($r = $modx->db->getRow($result))   
{ 
     $id = $r['id'];
     $createdby = $r['createdby'];
     $parent = $r['parent'];
      
      //if parent equals parentID (document container for news articles) and createdby equals uid print 
      if ($parent == $parentID && $createdby == $uid)
      {
            //list this document
            $articleListingForUser = $createdby . ',';
      }
}
//Trim any leading or trailing commas and spaces from the string of articles
$articleListingForUser = trim($articleListingForUser, " \,");

//Set the placeholder to display the number of articles the user has posted
$modx->setPlaceholder('articleListingForUser', "($articleListingForUser)");