im trying to call the method i made in function.php but undefined. i write it correctly though. still gets the fatal error, help me this is my 1st php

Fatal error: Uncaught Error: Call to undefined functionis_admin_login() in D:\xamapp\htdocs\LMS\header.php:47 Stack trace: #0D:\xamapp\htdocs\LMS\index.php(5): include() #1 {main} thrown inD:\xamapp\htdocs\LMS\header.php on line 47

this is the function.php

including header.php to index.php

calling the method is_admin_login in header.php