PHPStorm - 错误的根文件夹? 完整路径

I have just installed PHPStorm and i'm using MAMP on my mac as local server. I have created a PHP Project and wanted do require_once() to a file which is inside a folder called functions.

But when i want to refer to that file i can't just type "/" infront of it. I have type the full path like: /Applications/MAMP/htdocs/functions

What have i done wrong?

None of these is found:

require_once '/functions/sanitize.php';

require_once 'functions/sanitize.php';

But this one works fine:

require_once '/Applications/MAMP/htdocs/functions/sanitize.php';

Hope you can help me out.