How to debug "Notice: undefined Index" in a PHP website?

Question

<?php include('header.php'); 
if(isset($_POST['submit']))
{
    $key=$_POST['key'];
   $location=$_POST['location']; 
        ?>

         <script>
                  window.location.href = 'serach_job.php?key=<?php echo $key;?>&location=<?php echo $location;?>'; 
         </script>
     <?php  
}
?>
<div class="banner">
    <div class="container">
        <div id="search_wrapper">
       <?php if($_SESSION['member']['id']){ ?>      
         <form method="POST">
          <div id="search_form" class="clearfix">
          <h1>Start your job search</h1>
            <p>
             <input type="text" name="key" class="text" placeholder="Enter a keywords(s)"  >
             <input type="text" name="location" class="text" placeholder="Location"  >
             <label class="btn2 btn-2 btn2-1b"><input type="submit" name="submit" value="Find Jobs"></label>
           </p>

         </div>

         </form>


         <?php }?>

       </div>
   </div> 
</div>  
<div class="container">
<p style="margin-top: 30px;">Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.</p>

<p style="margin-top: 30px;">it is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout. The point of using Lorem Ipsum is that it has a more-or-less normal distribution of letters, as opposed to using 'Content here, content here', making it look like readable English. Many desktop publishing packages and web page editors now use Lorem Ipsum as their default model text, and a search for 'lorem ipsum' will uncover many web sites still in their infancy. Various versions have evolved over the years, sometimes by accident, sometimes on purpose (injected humour and the like).</p>
  <div class="grid_1">
     <h3>Featured Employers</h3>
       <ul id="flexiselDemo3">
          <li><img src="images/c1.gif"  class="img-responsive" /></li>
          <li><img src="images/c2.gif"  class="img-responsive" /></li>
          <li><img src="images/c3.gif"  class="img-responsive" /></li>
          <li><img src="images/c4.gif"  class="img-responsive" /></li>
          <li><img src="images/c5.gif"  class="img-responsive" /></li>
          <li><img src="images/c6.gif"  class="img-responsive" /></li>  
        </ul>
        <script type="text/javascript">
         $(window).load(function() {
            $("#flexiselDemo3").flexisel({
                visibleItems: 6,
                animationSpeed: 1000,
                autoPlay:false,
                autoPlaySpeed: 3000,            
                pauseOnHover: true,
                enableResponsiveBreakpoints: true,
                responsiveBreakpoints: { 
                    portrait: { 
                        changePoint:480,
                        visibleItems: 1
                    }, 
                    landscape: { 
                        changePoint:640,
                        visibleItems: 2
                    },
                    tablet: { 
                        changePoint:768,
                        visibleItems: 3
                    }
                }
            });

        });
       </script>
       <script type="text/javascript" src="js/jquery.flexisel.js"></script>
     </div>


</div>
<?php include("footer.php");?>

Here is the error:

Notice: Undefined index: member in C:\xampp\htdocs\recruitment\index.php on line 17

I am getting running on my index page of the website so kindly please help me how to get rid of all of these errors of undefined index