Login Script

Andaikan/ Suppose that
Database: whatever (Mysql Database)
Table: user
username: abcde (Mysql username)
password: aa123 (Mysql Password)

<form action="loginprocess.php" method="post">
<table width="302" height="175" border="1">
  <tr>
    <td>Username</td>
    <td><label>
      <input type="text" name="username">
    </label></td>
  </tr>
  <tr>
    <td>Password</td>
    <td><label>
      <input type="password" name="password">
    </label></td>
  </tr>
  <tr>
    <td>&nbsp;</td>
    <td><input type="submit" name="submit" value="Login"></td>
  </tr>
</table>
</form>

Username
Password

Proses login:
loginprocess.php
<?php
session_start();
mysql_connect('localhost','abcde','aa123') or die('could not connect to server');// sambungan ke server/ connect to server
mysql_select_db('whatever') or die ('could not connect to database');// sambungan ke database/ connect to database

if($_POST['submit']){ // sekiranya butang login diklik/ if submit button been clicked
      $result=mysql_query("SELECT * FROM user WHERE username='$_POST[username]' and password='$_POST[password]'");// query data daripada database/ query username and password from table user

      switch($result){   //menggunakan fungsi switch()/ using function switch
         case mysql_num_rows($result)==1: //sekiranya username dan password wujud pada satu row/ if username and password exist in one row
         $success=mysql_fetch_array($result);
         $user=$success['username'];
         $_SESSION['user']=$user;
         header('location:profile.php');
     break;
         case mysql_num_rows($result)==0:   //sekiranya username dan password salah atau tidak wujud pada satu row/ if username and 
password entered is invalid, then will be directed to invalid.php
         header('location:invalid.php');
     break;   
       }
}
?>
Atau/ Or: 
<?php
session_start();
mysql_connect('localhost','abcde','aa123') or die('could not connect to server');// sambungan ke server/ connect to server
mysql_select_db('whatever') or die ('could not connect to database');// sambungan ke database/ connect to database

if($_POST['submit']){ // sekiranya butang login diklik/ if submit button been clicked
      $result=mysql_query("SELECT * FROM user WHERE username='$_POST[username]' and password='$_POST[password]'");// query data daripada database/ query username and password from table user

   if(mysql_num_rows($result))==1){//using function if()
      $success=mysql_fetch_array($result);
      $user=$success['username']; 
      $_SESSION['user']=$user; //the session used for next pages. Username used as session here
     
      header('location:profile.php');
     }
   else
      header('location:invalid.php');
  }
?>

Note: 
on my script above, i used direct select from the database: SELECT * FROM user WHERE username='$_POST[username]' and password='$_POST[password]'".
For the security reasons, better put mysql_real_escape_string($_POST['username']; and mysql_real_escape_string($_POST['password']; to prevent sql injection.
for example:
$username= mysql_real_escape_string($_POST['username'];
$password= mysql_real_escape_string($_POST['password'];
 SELECT * FROM user WHERE username='$username' and password='$password'".