coba

<?php ob_start();

    #**************** koneksi ke mysql *****************#
   $host = "localhost";
    $user = "root";
    $pass = "";
    $dbname = "pt_pos";
    $conn = mysql_connect($host,$user,$pass);
    if($conn) {
    //select database
    $sele = mysql_select_db($dbname);
    if(!$sele) {
    echo mysql_error();
    }
    }
    #***************** akhir koneksi ******************#
   #jika ditekan tombol login
   if(isset($_POST['login'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    $sql = mysql_query("select id_karyawan,username,password from karyawan where username='$username' and password='$password'");
    $num = mysql_num_rows($sql);
    if($num==1) {
    // login benar //
    $_SESSION['username'] = $username;
    $_SESSION['password'] = $password;
  echo " <script language='JavaScript'>alert('Anda berhasil login')
   </script>
   halo berhasil";
    }
    else {
    // jika login salah //
   echo "<script language='JavaScript'>alert('Anda gagal login')
   document.location='log.php'</script>";
    }
    }

ob_end_flush();?>