CSS links are relative to the current request URI, not your PHP file. Use an absolute path like:
<link rel="stylesheet" href="https://wordpress.stackexchange.com/questions/184258/<?php bloginfo("template_url' ) ?>/themesubfolder/style.css" />
CSS links are relative to the current request URI, not your PHP file. Use an absolute path like:
<link rel="stylesheet" href="https://wordpress.stackexchange.com/questions/184258/<?php bloginfo("template_url' ) ?>/themesubfolder/style.css" />