First of all I try and avoid such situations.
When I can't, require_once("../includes/somefile.php"); works for me. But then I'm not sure what would happen if somefile.php would contain another include or require.
While I would prefer ASP.Net, PHP has the advantage of being able to be hosted on Linux servers. This keeps running costs much lower than ASP.Net servers. Functionally they are pretty much the same, PHP has some funny syntax. ASP.Net has the advantage of being able to use C# as code behind, which would integrate better with desktop applications and WCF(web) services. The choice is ultimately just an arbitrary one, dependant on the developer.
Having recently moved a site from ASP to PHP I can tell you that ASP was 10x easier for me (but most of my exp until recently in programming has been VB, C#). I was able to get things working a lot faster in ASP and how I wanted them to. Off hand I can't really think of anything I have done in PHP that I thought, "God I'm sure glad I'm using PHP to do this, as it's so much easier".
The only up side is the use of a Linux server for PHP. I'm sure there are other pros and cons, but I work on a web site about 3 hours a month for a intranet site here at work and now just use PHP as I'm forced to (admin won't run IIS).
$path_root = $_SERVER['DOCUMENT_ROOT'].'/';
include($path_root.'aaa.php'); // this one works well
include($path_root.'bbb.php'); // this one gives me the error * (see below).
* Error: : "Warning: include(C:/Program Files/EasyPHP-184.108.40.206/wwwbbb.php) [function.include]: failed to open stream: No such file or directory in C:\Program Files\EasyPHP-220.127.116.11\www\index.php on line 9
It seems to me that, for any unknown reason the last '/' is getting removed automatically after using it inside an include clause.
why do you feel a need to specify an explicit path in an include?
Probably this is again a bug then... I've tried to use relative paths like '../aaa.php' and so on, but the problem here is that I'm using some files from different locations and those files seem to inherit (somehow) the original path... then the relative paths change depending on the file that is including the file that includes another file... Using the doc_root helps me avoiding this problem...