How to add new CSS file to new PHP file

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" />

techhipbettruvabetnorabahisbahis forumutaraftarium24eduedueduseduseduedusedueduedusedu