Errors are not showing up

1. What is this talk about PHP 5.3? I did find it was added 13 years ago https://git.drupalcode.org/project/filefield_sources/-/commit/57a0d0a643...
I think this might be replaced by what is in file.module

2. All t() functions return object so doing .= will fail.

3. $file->filename is not string but but object. I think something like $file->getFileName() should be used.

/**
 * Validate a file based on the $element['#upload_validators'] property.
 */
function filefield_sources_element_validate($element, $file, FormStateInterface $form_state) {
  $validators = $element['#upload_validators'];
  $errors = [];

  // Since this frequently is used to reference existing files, check that
  // they exist first in addition to the normal validations.
  if (!file_exists($file->getFileUri())) {
    $errors[] = t('The file does not exist.');
  }
  // Call the validation functions.
  else {
    foreach ($validators as $function => $args) {
      // Add the $file variable to the list of arguments and pass it by
      // reference (required for PHP 5.3 and higher).
      array_unshift($args, NULL);
      $args[0] = &$file;
      $errors = array_merge($errors, call_user_func_array($function, $args));
    }
  }

  // Check for validation errors.
  if (!empty($errors)) {
    $message = t('The selected file %name could not be referenced.', ['%name' => $file->filename]);
    if (count($errors) > 1) {
      $message .= '<ul><li>' . implode('</li><li>', $errors) . '</li></ul>';
    }
    else {
      $message .= ' ' . array_pop($errors);
    }
    $form_state->setError($element, $message);
    return 0;
  }

  return 1;
}

Closing as a duplicate of 🐛 Make sure $callback is_callable. ( TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function "FileSizeLimit" not found or invalid function name ) Needs review . There are two different solutions there that need community review.