title: Notice Undefined Index
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/4261133/php-notice-undefined-variable-and-notice-undefined-index">PHP: “Notice: Undefined variable”  and “Notice: Undefined index”</a><br>
    <a href="https://stackoverflow.com/questions/12769982/reference-what-does-this-error-mean-in-php">Reference - What does this error mean in PHP?</a>  </p>
  </blockquote>
  
  
  
  <blockquote>
    <p>Notice: Undefined index: ac in C:\xampp\htdocs\CMS\includes\login.php
    on line 6</p>
    
    <p>Notice: Undefined index: logged in
    C:\xampp\htdocs\CMS\includes\login.php on line 13</p>
  </blockquote>
  
  <p>I am receiving the above Notices with the below code. How can I define my indexes? I understand there is a way to use ISSET but I am not sure how to do it with $_SESSION logged and USERS; since there are multiple values. How would I clear up the above errors in a correct manner without just suppressing notices/errors?</p>
  
  <pre><code>&lt;?php
  session_start(); // initialize session
  include($_SERVER['DOCUMENT_ROOT'] . 
          '/CMS/CMSController.php');
  
  if ($_POST["ac"]=="log") { /// do after login form is submitted  
       if ($USERS[$_POST["username"]]==$_POST["password"]) { /// check if submitted username and password exist in $USERS array 
            $_SESSION["logged"]=$_POST["username"]; 
       } else { 
            echo 'Incorrect username/password. Please, try again.'; 
       }; 
  }; 
  if (array_key_exists($_SESSION["logged"],$USERS)) { //// check if user is logged or not  
      echo "You are logged in.";
      header('Location: /CMS/index.php');
  } else { //// if not logged show login form 
       echo '&lt;form action="login.php" method="post"&gt;&lt;input type="hidden" name="ac" value="log"&gt; '; 
       echo 'Username: &lt;input type="text" name="username" /&gt;'; 
       echo 'Password: &lt;input type="password" name="password" /&gt;'; 
       echo '&lt;input type="submit" value="Login" /&gt;'; 
       echo '&lt;/form&gt;'; 
  }; 
  ?&gt;
  </code></pre>