header - Wordpress: Call to undefined function get_header(),get_header() run in index.php but this not run in amthuc.php -

i meet error,that not index.php, error in page include index, name amthuc.php. can me? please, :) get_header() run in index.php not run in amthuc.php

fatal error: call undefined function get_header() in /home/content/p3pnexwpnas03_data01/11/2436011/html/wp-content/themes/pnac/amthuc.php on line 1