$ _POST给出未识别的索引错误[重复]

I am making an ajax login script and my html code is as follows :

<form class="form-container" action="login.php" method="POST">
  <div class="form-title"><h2>Sign up</h2></div>
  <div class="form-title">Name</div>
  <input class="form-field" type="text" name="username" /><br />
  <div class="form-title">Password</div>
  <input class="form-field" type="password" name="pass" /><br />
  <div class="submit-container">
    <input class="submit-button" type="submit" value="Submit" />
  </div>
</form>

and my php code is as follows:

$client_username = $_POST["username"];
$client_password = $_POST["pass"]

when i run the code, i get an error in wamp server that says Unidentified Index in my php code. What is wrong and how can i fix it?

EDIT :

I tried this but in vain :

if(isset($_POST["username"]))
{
     $client_username = $_POST["username"];
}

if(isset($_POST["pass"]))
{
  $client_password = $_POST["pass"];
}


echo $client_username;

echo $client_password;

</div>

Try this, just use isset() to check the POST variables

$client_username = "";
$client_password = "";

if(isset($_POST["username"]));
{
     $client_username = $_POST["username"];
}

if(isset($_POST["pass"]))
{
   $client_password = $_POST["pass"];
}