fix Cannot redeclare

Cannot redeclare function() (previously declared in...
when using include
when a page is called, a file called a.php is included to check if the username and password of the user are correct. This file includes b.php. However, I also included a.php in the file after including b.php.
who has this problem, change require(...); or include(...); to require_once(...); or include_once(...);.