Login using session with php and mysql

Code:

Authentication.php:

<?php 
      include 'header.php';
	  
	  
          function Login($username, $password)
   {
     $query=" SELECT * FROM Account where username='$username' and password='$password'";
	 $result=mysql_query($query);
	 echo mysql_error();
	 $num=mysql_num_rows($result);
	  if($num==0)
	  {
	   return false;
	  }
	  else
	  return true;
 
   
   }
   
   $username=$_POST['username'];
   $password=$_POST['password'];
   
   if(Login($username,$password))
    {
	   $_SESSION['username']=$username;
	   echo "Welcome ".$username ;
	   ?>
	    <a href="index.php" >Home</a>
	   <?
	   
	}
	else 
	{
	  echo "<center><b> Wrong username and password </b></center> " ; 
	}
	
	


?>

Code:

Config.php

<?php
$servername='localhost';     // Your MySql Server Name or IP address here
$dbusername='root';                // Login user id here
$dbpassword='';                // Login password here
$dbname='IBM';     // Your database name here
global $link;
connecttodb($servername,$dbname,$dbusername,$dbpassword);


function connecttodb($servername,$dbname,$dbuser,$dbpassword)
{

$link=mysql_connect ("$servername","$dbuser","");
if(!$link){die("Could not connect to MySQL");}
mysql_select_db("$dbname",$link) or die ("could not open db".mysql_error());
}


?>

Code:

Login.php

<? 
  include 'header.php';
  
 ?>
 
<form action="Authentication.php" method="post">


<b>Username : </b><input type="text" name="username" size="30" /><br/>
<b>Password : </b><input type="password" name="password" size="30" />
<input type="hidden" name="login" value="x1" />
<input type="submit" value="Login" />


</form>


<?
    
     
  
 
?>

Code:

logout.php

<? session_start();

   unset($_SESSION['username']);
?>

Code:

Header.php

<?php   session_start();
    include "config.php"; 
	
	 if(!isset($_SESSION['username']))
	 {
	    ?>
		 <a href="Login.php" />Login </a>
		<?
	 }
	 else 
	 {
	  echo "welcome ".$_SESSION['username'];
	  
	  ?>
	   <a href="Logout.php" />Logout </a>
	  <?
	 }

?>

Code:

index.php

<? 
  include "Header.php";
  
  
  ?>