open file in registered shutdown function php -

i want log information when script ends, thought of using register_shutdown_function, can explain me why works:

<?php //register_shutdown_function('on_exit'); on_exit(); //function on_exit called function on_exit(){ $path = './myfolder/myfile'; $fo = fopen($path,'w+'); fwrite($fo,"test"); fclose($fo); } ?> 

and not:

<?php register_shutdown_function('on_exit'); //function on_exit called shutdown function //on_exit(); function on_exit(){ $path = './myfolder/myfile'; $fo = fopen($path,'w+'); fwrite($fo,"test"); fclose($fo); } ?> 

returning

warning: fopen(./myfolder/myfile): failed open stream: no such file or directory in c:\xampp\htdocs\test.php on line 6  warning: fwrite() expects parameter 1 resource, boolean given in c:\xampp\htdocs\test.php on line 7  warning: fclose() expects parameter 1 resource, boolean given in c:\xampp\htdocs\test.php on line 8