Custom next & prev post link

custom-link-post.php

// Add custom class to link NEXT & PREV
function popy_add_class_next_post_link($html){
  $html = str_replace('<a','<a class="icon-next"',$html);
	return $html;
}
add_filter('next_post_link','popy_add_class_next_post_link',10,1);

function popy_add_class_previous_post_link($html){
	$html = str_replace('<a','<a class="icon-prev"',$html);
	return $html;
}
add_filter('previous_post_link','popy_add_class_previous_post_link',10,1);