How to override Joomla JHtml functions

There are many pages out there explaining how to override Joomla Layout, or even full classes, but it's also possible to overrides functions called by JHtml::_() function.

To do that, all you need to do, is to place the function to override in a system plugin, and to register the function override in a onAfterInitialise() function.

here is an example (that forces the date picker widget to use Monday as the first day of the week):

<?php
/**
 * @package     Mypackage
 * @subpackage  Plugins
 *
 * @license     GNU General Public License version 2 or later, see LICENSE.
 */

// No direct access
defined('_JEXEC') or die('Restricted access');

// Import library dependencies
jimport('joomla.plugin.plugin');

use Joomla\Utilities\ArrayHelper;

/**
 * Jhtml override
 *
 * @since  1.0.0
 */
class PlgSystemJhtmloverride extends JPlugin
{
   public function onAfterInitialise()
   {
      JHtml::register('calendar', array('PlgSystemJhtmloverride', 'calendar'));
   }

   /**
    * Displays a calendar control field
    *
    * @param   string  $value    The date value
    * @param   string  $name     The name of the text field
    * @param   string  $id       The id of the text field
    * @param   string  $format   The date format
    * @param   mixed   $attribs  Additional HTML attributes
    *
    * @return  string  HTML markup for a calendar field
    *
    * @since   1.5
    */
   public static function calendar($value, $name, $id, $format = '%Y-%m-%d', $attribs = null)
   {
      static $done;

      if ($done === null)
      {
         $done = array();
      }

      $readonly = isset($attribs['readonly']) && $attribs['readonly'] == 'readonly';
      $disabled = isset($attribs['disabled']) && $attribs['disabled'] == 'disabled';

      if (is_array($attribs))
      {
         $attribs['class'] = isset($attribs['class']) ? $attribs['class'] : 'input-medium';
         $attribs['class'] = trim($attribs['class'] . ' hasTooltip');

         $attribs = ArrayHelper::toString($attribs);
      }

      JHtml::_('bootstrap.tooltip');

      // Format value when not nulldate ('0000-00-00 00:00:00'), otherwise blank it as it would result in 1970-01-01.
      if ($value && $value != JFactory::getDbo()->getNullDate() && strtotime($value) !== false)
      {
         $tz = date_default_timezone_get();
         date_default_timezone_set('UTC');
         $inputvalue = strftime($format, strtotime($value));
         date_default_timezone_set($tz);
      }
      else
      {
         $inputvalue = '';
      }

      // Load the calendar behavior
      JHtml::_('behavior.calendar');

      // Only display the triggers once for each control.
      if (!in_array($id, $done))
      {
         $document = JFactory::getDocument();
         $document
            ->addScriptDeclaration(
               'jQuery(document).ready(function($) {Calendar.setup({
         // Id of the input field
         inputField: "' . $id . '",
         // Format of the input field
         ifFormat: "' . $format . '",
         // Trigger for the calendar (button ID)
         button: "' . $id . '_img",
         // Alignment (defaults to "Bl")
         align: "Tl",
         singleClick: true,
         firstDay: 1
         });});'
            );
         $done[] = $id;
      }

      // Hide button using inline styles for readonly/disabled fields
      $btn_style = ($readonly || $disabled) ? ' style="display:none;"' : '';
      $div_class = (!$readonly && !$disabled) ? ' class="input-append"' : '';

      return '<div' . $div_class . '>TEST'
         . '<input type="text" title="' . ($inputvalue ? JHtml::_('date', $value, null, null) : '')
         . '" name="' . $name . '" id="' . $id . '" value="' . htmlspecialchars($inputvalue, ENT_COMPAT, 'UTF-8') . '" ' . $attribs . ' />'
         . '<button type="button" class="btn" id="' . $id . '_img"' . $btn_style . '><span class="icon-calendar"></span></button>'
         . '</div>';
   }
}

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