PHP cookie management is a good example of how incredibly badly designed PHP is:

Set a cookie that expires when browser is closed

Note that you most probably want to set the path, which we do here:

$value = 0;
$expires = 0;
setcookie('cookie_name', $value, $expires, '/');

Remove cookie

setcookie ('cookie_name', FALSE, time()-10000);

Get cookie value

$_COOKIE['cookie_name']

This snippet shows how to change the jQuery validation plugin's validation rules dynamically:

$(document).ready(function() { 
  // Setup the default validation rules
  var validator = $('form').validate({ 
    rules: { 
      username: { 
        required: true, 
        minlength: 2
      }
    }
  };

  // Drop-down selection changes validation rules
  $('#account-type').change(function () {
    // Get the jQuery validation plugin's settings
    var settings = $('form').validate().settings;

    // Modify validation settings
    $.extend(true, settings, {
       rules: {
           // Add validation of year
           "year": {
               required: true,
               min: 2010,
               max: 2020,
               number: true
           }, 
           // Remove validation of username
           "username": {} 
       }
    });
  });
});

You can also use CSS classes to achieve the same:

$('#channel-title').addClass("{required:true,messages:{required:'required field'}}")
$('#channel-title').removeClass("{required:true,messages:{required:'required field'}}")