// Include Jomsocial core
$jspath = JPATH_ROOT.DS.'components'.DS.'com_community';
include_once($jspath.DS.'libraries'.DS.'core.php');
// Include Messaging library
include_once($jspath.DS.'libraries'.DS.'messaging.php');
// Add onclick action
$onclick = CMessaging::getPopup($userid);
echo '<a onclick="<?php echo $onclick; ?>" href="javascript:void(0);">';

Use JomSocial built-in personal messaging system

joomla

PHP