Add template function to header

Hi, I'm trying to add a template function to the header of my child theme. The code relates to a plugin which inserts icons, either by means of a widget or by inserting a function in the header. (I know the Box Catch theme has it's own social media feature but I'd prefer to use this system as I've installed it on other sites). In the Box Catch theme the widget works which means that the plugin is compatible; however I want to insert the icons in the header and they do not appear when I add the function. I've already used the header method in another theme so there doesn't seem to be a problem with the function itself. <php if (function_exists('synved_social_follow_markup')) echo synved_social_follow_markup(); ?> Is it possible that the code isn't compatible with HTML5? Or could you tell me please if there's some other explanation?