Display error messages and notices
From Joomla! Documentation
Errors, warnings and notices can be displayed from any component, module, plugin or template using the methods outlined below.
//Get a handle to the Joomla!-application object
$app =& JFactory::getApplication();
//add a message to the message queue
$app->enqueueMessage(JText::_('Random error occured'), 'error');
/** PHP 5 */
JFactory::getApplication()->enqueueMessage(JText::_('Random error occured'), 'error');The second argument to the enqueueMessage()-function is the type of message. Default is 'message', but 'error' gives a different style message. Try other types as well, to see how that works. The api for the JApplication-object can be found at: JApplication API The error message will now be displayed if the message-tag is present in your template. The message-field is added with the following statement in your template:
<jdoc:include type="message" />Message[edit]
Message
/** PHP 4 */
$app = JFactory::getApplication()
$app->enqueueMessage('Message');
/** PHP 5 */
JFactory::getApplication()->enqueueMessage('Message');Notice[edit]
Notice
JError::raiseNotice(100, 'Notice');Warning[edit]
Warning
JError::raiseWarning(100, 'Warning');Error[edit]
JError::raiseError(4711, 'A severe error occured');