在导出SQL数据库时，将utf8列中的数据乱码

我使用了PongoCMS中的db_backup函数对MySQL数据库进行了备份。几乎所有的列在所有的数据库表导出正确，但在表列与'utf8'编码(我已经使用utf8_unicode_ci排序)，该列的导出数据填充问号。

public static function db_backup()
{
$type_db = Config::get('database.default');
$connections = Config::get('database.connections');
switch($type_db) {
  case 'sqlite':
    $file_name = $connections[$type_db]['database'] . '.' . $connections[$type_db]['driver'];
    break;
  case 'mysql':
    $link = mysql_connect($connections[$type_db]['host'],$connections[$type_db]['username'],$connections[$type_db]['password']);
    mysql_select_db($connections[$type_db]['database'],$link);
    $tables = array();
    $result = mysql_query('SHOW TABLES');
    while($row = mysql_fetch_row($result))
    {
      $tables[] = $row[0];
    }
    //Set time now
    $now = date('Y-m-d-H-i-s');
    //File header
    $return ="### DB BACKUP: " . $connections[$type_db]['database'] . " at " . $now . " ###'n'n'n";
    //cycle through
    foreach($tables as $table)
    {
      $result = mysql_query('SELECT * FROM '.$table);
      $num_fields = mysql_num_fields($result);
      $return.= 'DROP TABLE IF EXISTS '.$table.';';
      $row2 = mysql_fetch_row(mysql_query('SHOW CREATE TABLE '.$table));
      $return.= "'n'n".$row2[1].";'n'n";
      for ($i = 0; $i < $num_fields; $i++)
      {
        while($row = mysql_fetch_row($result))
        {
          $return.= 'INSERT INTO '.$table.' VALUES(';
          for($j=0; $j<$num_fields; $j++)
          {
            $row[$j] = addslashes($row[$j]);
            $row[$j] = preg_replace("#'n#i","''n",$row[$j]);
            if (isset($row[$j])) { $return.= '"'.$row[$j].'"' ; } else { $return.= '""'; }
            if ($j<($num_fields-1)) { $return.= ','; }
          }
          $return.= ");'n";
        }
      }
      $return.="'n'n'n";
    }
    //save file
    $file_name = 'db-backup-'.$now.'.sql';
    $handle = fopen(path('storage') . 'database/' . $file_name, 'w+');
    fwrite($handle, utf8_encode($return));
    fclose($handle);
}
return $file_name;
}

但是，如果从phpmyadmin导出数据库，则数据库导出正常。在上述函数中应该更改/添加什么，以便该函数也正确导出数据库?