require_once differs functionality from require

The following will work

if ($loggedIn = $sess->is_registered("auth"));
require ($loggedIn) ? 'header.inc': 'header_login.inc';

but if require is substituted with require_once, it will error with:
Parse error: parse error, expecting `T_CONSTANT_ENCAPSED_STRING' in...

Is this functionality correct, that is, is require_once more precise about 
what it does than require, or is this a bug?

Thanks,
Tom



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