edit

Extending Smarty

The Smarty templating language that is used to output the HTML for CiviCRM's templates is fairly powerful in its own right, but sometimes it can't do everything you want. Smarty can be extended using "plugins" -- you can find some examples and documentation online at Smarty's website or by searching the web.

To install Smarty plugins without editing CiviCRM core (which is difficult to maintain), you will have to implement hook_civicrm_config.

Once you've created your module or hook file, you can retrieve the Smarty object and register your custom plugin path:

function yourmodule_civicrm_config(&$config) {
  $smarty = CRM_Core_Smarty::singleton();
  array_push($smarty->plugins_dir, __DIR__ . '/relative/path/to/custom/plugin/directory');
}

Then in that custom plugin directory, you can place whatever Smarty plugins you need.

You can also use this trick to change other Smarty behavior, such as whether it can evaluate PHP placed directly in templates. For instance:

function yourmodule_civicrm_config(&$config) {
  $smarty = CRM_Core_Smarty::singleton();

  // allow the explode() php function to be used as a "modifier" in Smarty templates
  array_push($smarty->security_settings['MODIFIER_FUNCS'], 'explode'); 
}

However, be very careful with these settings – if you don't know what you're doing, you can open security holes or create a mess of code that's difficult to maintain.