Login using MD5 in PHP

<form class="form-horizontal" method="POST">
   <div class="control-group">
      <label class="control-label" for="inputEmail">Username</label>
      <div class="controls">
         <input type="text" id="inputEmail" name="username" placeholder="Username" required>
      </div>
   </div>
   <div class="control-group">
      <label class="control-label" for="inputPassword">Password</label>
      <div class="controls">
         <input type="password" name="password" id="inputPassword" placeholder="Password" required>
      </div>
   </div>
   <div class="control-group">
      <div class="controls">
         <button type="submit" name="login" class="btn btn-primary">Login</button>
      </div>
      <br>
      <?php
         if (isset($_POST['login'])){
         $username=$_POST['username'];
         $password=md5($_POST['password']);
 
         $login=mysql_query("select * from user where username='$username' and password='$password'")or die(mysql_error());
         $count=mysql_num_rows($login);
 
         if ($count > 0){
         header('location:home.php');
         }else{ ?>
      <div class="alert alert-error">Error login! Please check your username or password</div>
      <?php
         }}
         ?>
   </div>
</form>