To load a CSS file into a Joomla PHP page.
Defining URL of the CSS file in $url variable...
$url = "components/com_mycomponent/assets/css/style.css";
Loading $url CSS file...
$document = JFactory::getDocument();
N.B.: It used to be possible with JHTML. However, this was deprecated in Joomla 2.5 and removed in Joomla 3.x.