<p class="normal-welcome">This is the normal welcome text...<p>
<p class="survey-full-welcome" style="display:none">Sorry, this survey is full.<p>

function handleCompletes(sid, maxCount) {
  // Identify some stuff...
  var surveyRoot = location.pathname.split('index.php')[0];
  var templateName = $('head link[href*="template.css"]').attr('href').replace(/\/template.css/, '').split('/templates/')[1];
  var templatePath = surveyRoot+'upload/templates/'+templateName+'/';
 
  // AJAX call
  $.ajax({
    url: templatePath+'db_query.php', // The PHP file
    async: true,
    cache : false, // No caching in IE
    data: {
      sid: sid // Send the survey ID to the PHP file
    },
    success: function(results){
      // If we're at  or above the maximum completes
      if($.isNumeric(results) && results >= maxCount) {
        // Handle the welcome text elements
        $('.normal-welcome').hide();
        $('.survey-full-welcome').show();
        // Remove all of the buttons
        $('#movenextbtn, #loadallbtn, #clearall, #therearexquestions').remove();
 
        // Redirect here if you want
      }
    },
    error: function(){
      alert('Could not connect!');
    }
  }); 
}

<script type="text/javascript" charset="utf-8">    
  $(document).ready(function(){
    handleCompletes({SID}, 10);
  });
</script>

<?php
 
  define( 'LS_DB_HOST', 'localhost' );
  define( 'LS_DB_NAME', 'database_name' );
  define( 'LS_DB_USER', 'database_user_name' );
  define( 'LS_DB_PASS', 'database_user_password' );
  define( 'LS_DB_TABLEPREFIX', 'lime_' );
 
  $sid = $_GET["sid"];
 
  if(ctype_alnum($sid) &amp;&amp; strlen($sid) == 6) { // Valid SID format
 
    $link = mysqli_connect(LS_DB_HOST, LS_DB_USER, LS_DB_PASS, LS_DB_NAME);
 
    // Check the connection 
    if (mysqli_connect_errno()) {
      printf("Connect failed: %s\n", mysqli_connect_error());
      exit();
    }
 
    // Query the survey table for all completed responses
    if ($result = mysqli_query($link, "SELECT * FROM ".LS_DB_TABLEPREFIX."survey_".$sid." WHERE `submitdate` IS NOT NULL")) {
 
      // Count number of rows in the result */
      $completedCount = mysqli_num_rows($result);
 
      // Return that value
      echo $completedCount;
 
      // Close result set 
      mysqli_free_result($result);
    }
 
    // Close the connection 
    mysqli_close($link);
 
  }
  else { // Invalid SID format
    die( 'Invalid format!' );
  }
 
?>

<?php
 
  require_once 'jsonRPCClient.php';
 
  define( 'LS_BASEURL', 'http://path/to/limeSurvey/');  
  define( 'LS_USER', 'limesurvey_user_name' );
  define( 'LS_PASSWORD', 'limesurvey_user_password' );
  define( 'LS_DB_HOST', 'localhost' );
  define( 'LS_DB_NAME', 'database_name' );
  define( 'LS_DB_USER', 'database_user_name' );
  define( 'LS_DB_PASS', 'database_user_password' );
  define( 'LS_DB_TABLEPREFIX', 'lime_' );
 
  $sid = $_GET["sid"];
 
  if(ctype_alnum($sid) &amp;&amp; strlen($sid) == 6) { // Valid SID format
 
    // Instantiate a new RPC client
    $myJSONRPCClient = new jsonRPCClient( LS_BASEURL.'/index.php/admin/remotecontrol' );
 
    // Get a session key
    $sessionKey= $myJSONRPCClient->get_session_key( LS_USER, LS_PASSWORD );
 
    // Call the API with the get_summary() method
    $completedCount = $myJSONRPCClient->get_summary($sessionKey, $sid, 'completed_responses');
 
    // Return the completed surveys count
    echo $completedCount;
 
    // Release the session key
    $myJSONRPCClient->release_session_key( $sessionKey );
  }
  else { // Invalid SID format
    die( 'Invalid format!' );
  }
 
?>