<form action="logindo.php" method="POST">
    <table border="1">
      <tr bgcolor="#993399"> 
        <td colspan="2"> 
          <div align="center"><font color="#FFFFFF">Login</font></div></td>
      </tr>
      <tr> 
        <td width="114">Matrix Number</td>
        <td width="174">: 
          <input name="matrix_no" type="text" id="matrix_no" /></td>
      </tr>
      <tr> 
        <td>Password</td>
        <td>: 
          <input type="password" name="password" /> <input name="submit" type="submit" value="send" /></td>
      </tr>
    </table>
    [ <a href="register.php">register</a>? ] 
  </form>

<?php 
session_start(); 
error_reporting(E_ALL); 

// Connect to the database server 
$dbcnx = mysql_connect('localhost', 'root', '') 
             or die('<p>Unable to connect to the database server at this time.<br />Error: ' . mysql_error() . '</p>'); 

// Select the huh database 
mysql_select_db('profile') 
             or die('<p>Unable to locate the profile database at this time.<br />Error: ' . mysql_error() . '</p>'); 

$matrix_no = strip_tags(trim($_POST['matrix_no'])); 
$password = strip_tags(trim($_POST['password'])); 

$sql = "SELECT matrix_no, password, user_id 
          FROM userinfo 
         WHERE matrix_no = '$matrix_no' 
           AND password = '$password' 
         LIMIT 0,1"; 

$result = mysql_query($sql) 
             or die('<p>Unable to query the profile database at this time.<br />Error: ' . mysql_error() . '</p>'); 

$row = mysql_fetch_array($result); 
echo '<p>sql: ' , $sql , '<br />Matrix Number: ' , $row['matrix_no'] , '<br />Password: ' , $row['password'] , '</p>'; 

if(mysql_num_rows($result) == 1) // mysql_num_rows() returns the record count from the query result.  you want one (1) matching record to verify the login was successful. 
{ 
     $_SESSION['loggedin'] = 1; // Setting session var 'loggedin' to true 
	 $_SESSION['user_id'] = $user_id;
     header('Location:member.php'); 
     exit; 
} 
else 
{ 
     header('Location:error.php'); // Redirect to error page, as a matching record was not found in the table. 
     exit;  // always 'exit' 
} 
?>

$sql="UPDATE userinfo SET hostel = '$hostel' WHERE matrix_no = '{$_SESSION['matrix_no']}'";

$sql="UPDATE userinfo SET hostel = '$hostel' WHERE matrix_no = $_SESSION[matrix_no]";