อยากทราบวิธี Import / Export to File .Sql โดยใช้ PHP CODE

$db=new mysqli('host', 'user', 'psw', 'dbname');
$rs = $db->query('select * from tablename');
$f = fopen('file.sql', 'w');
while( $ro = $rs->fetch_row()){
     $txt = '';
     foreach($ro as $value){
          $txt .= ($txt? ',' : '') . "'".$db->real_escape_string($value)."'";
     }
     fwrite($f, "insert into tablename values( $txt );\n";
}