title: How to pass the server side validation messages back to client?
body: <p>I have the following two script files (i.e., formtest.html and calc.php).
  When I do the server side validation on calc.php, how do I pass the error message (i.e. 
          $err_msg back to the formtest.html?</p>
  
  <p>Thank you</p>
  
  <pre><code>&lt;html&gt;
      &lt;head&gt;
          &lt;title&gt;Form Test&lt;/title&gt;
      &lt;/head&gt;
      &lt;body&gt;
  
      &lt;form method="post" action="calc.php"&gt;
      &lt;pre&gt;
            Loan Amount &lt;input type="text" name="principle" /&gt;
                        &lt;input type="submit" /&gt;
      &lt;/pre&gt;
      &lt;/form&gt;
      &lt;/body&gt;
      &lt;/html&gt;
  </code></pre>
  
  <p>.</p>
  
  <pre><code>// calc.php
  
          $err_msg = ''; 
  
          if ( !empty(isset($_POST['principle'])) )
          {
              // process the form and save to DB
          } else {
           $err_msg .= 'Loan Amount is empty!';
          }
  
          ?&gt;
  </code></pre>