What can cause a web server to not find an XML file?

Question

I have a PHP script which reads from an xml file to generate a table. The first thing it does is check to make sure the file exists.

$path = getcwd();
if(file_exists($path.'\inc\php\kbs.xml')){
    $kbs = simplexml_load_file($path.'\inc\php\kbs.xml');
} else {
    echo "Error: No KB file found";
}

For some reason, intermittently, it doesn't find the file. I've tried removing the file_exists check all together (since I know the file does exist) but it still doesn't load the file at times. I can refresh the page and 7 times out of 10 it doesn't load, but sometimes it does.

I never ran into this problem during development, but once it went production (being used by maybe 200 users now) it started happening.

How do I go about troubleshooting this? (PHP 5.2.14 running on IIS)

EDIT: Error logs give me the following messages when it fails:

Notice: Undefined variable: kbs in <the path> on line 16
Notice: Trying to get property of non-object in <the path> on line 16
Warning: Invalid argument supplied for foreach() in <the path> on line 16

line 16 is the first time the variable $kbs is accessed. Obviously $kbs isn't set if the file isn't found.

Answer 1

请使用绝对路径，相对路径会使事情变得一团糟。

Answer 2

Is the location relative to PHP? Do permissions allow the web server to see it?