Setting values of extened field using a postHook

i would like to name the file the same as the username and also put it in a folder with the name of the username too so i can pull these at a later date with the user profiles.

 $userName = $modx->getLoginUserName();

the line below doesnt 

<?php
// initialize output;

  
$output = true;

// valid extensions
$ext_array = array('pdf', 'txt', 'doc', 'docx', 'rtf', 'jpg',);

// create unique path for this form submission
  $userName = $modx->getLoginUserName();
  $uploadpath = 'assets/uploads/$userName';
 

// you can create some logic to automatically
// generate some type of folder structure here.
// the path that you specify will automatically
// be created by the script if it doesn't already
// exist.

// EXAMPLE:
// this would put all file uploads into a new,
// unique folder every day.
// $uploadpath = 'assets/'uploads/'.date('Y-m-d').'/';

// get full path to unique folder
$target_path = $modx->config['base_path'] . $uploadpath;

// get uploaded file names:
$submittedfiles = array_keys($_FILES);

// loop through files
foreach ($submittedfiles as $sf) {

  // Get Filename and make sure its good.
  $filename = basename( $_FILES[$sf]['name'] );

  // Get file's extension
  $ext = pathinfo($filename, PATHINFO_EXTENSION);
  $ext = mb_strtolower($ext); // case insensitive

  // is the file name empty (no file uploaded)
  if($filename != '') {
      
    // is this the right type of file?
    if(in_array($ext, $ext_array)) {
   
      // clean up file name and make unique
      $filename =  $userName($filename); // to lowercase
      $filename = str_replace(' ', '_', $filename); // spaces to underscores
      $filename = date("Y-m-d_G-i-s_") . $filename; // add date & time
      
      // full path to new file
      $myTarget = $target_path . $filename;
      
      // create directory to move file into if it doesn't exist
      mkdir($target_path, 0755, true);
      
      // is the file moved to the proper folder successfully?
      if(move_uploaded_file($_FILES[$sf]['tmp_name'], $myTarget)) {
        // set a new placeholder with the new full path (if you need it in subsequent hooks)
        $modx->setPlaceholder('fi.'.$sf.'_new', $myTarget);
        // set the permissions on the file
        if (!chmod($myTarget, 0644)) { /*some debug function*/ }
        
          } else {
        // File not uploaded
        $errorMsg = 'There was a problem uploading the file.';
        $hook->addError($sf, $errorMsg);
        $output = false; // generate submission error
      }
      
      } else {
      // File type not allowed
          $errorMsg = 'Type of file not allowed.';
      $hook->addError($sf, $errorMsg);
      $output = false; // generate submission error
        }
  
  // if no file, don't error, but return blank
  } else {
      $hook->setValue($sf, '');
  }

}

$userName = $modx->user->get('username');