I was having problems with a PHP script and, after a little research, I've found a bug in the web server related to variables submited in forms using POST method (specifically, when mixing POST method with variables in the URL, as in "www.myweb.com?variable=value").
Symptoms:

*) If you use GET to pass values to another page, everything works as expected: the variables submited via input fields in the form are passed to the target page, and if you try to add some variables to the target URL (with "?") they won't work -- i.e. they will be ignored.
*) If you use POST your input field variables will work just fine, BUT the variables passed using "?" in the URL won't get to the target page -- they will be erroneously ignored.

I put together a very simple PHP script (test_get_post.php) to test it:

<?php
	echo "<hr>";

echo "Values submitted via GET method:<br>";
	reset ($HTTP_GET_VARS);
	while (list ($key, $val) = each ($HTTP_GET_VARS)) {
	    echo "$key => $val<br>";
	}

echo "<hr>";

echo "Values submitted via POST method:<br>";
	reset ($HTTP_POST_VARS);
	while (list ($key, $val) = each ($HTTP_POST_VARS)) {
	    echo "$key => $val<br>";
	}
    echo "<hr>";

?>	
    <form name="test_post" method="post" action="test_get_post.php?variable0=Value for variable 0">
	    <input type="text" name="variable1" value="Value for variable 1">
	    <br>
	    <input type="text" name="variable2" value="Value for variable 2">
		<input type="submit" value="Submit with POST">    
    </form>

When executed under IIS, the results are:
Pressing the "Submit wiht POST" button:

--------------------------------------------------------------------------------
Values submitted via GET method:
variable0 => Value for variable 0
--------------------------------------------------------------------------------
Values submitted via POST method:
variable1 => Value for variable 1
variable2 => Value for variable 2
--------------------------------------------------------------------------------

As you can see, when using POST you can still use the "?" method to pass variable0 as a GET variable.
The same code under Abyss Web Server:
Pressing the "Submit wiht POST" button:

--------------------------------------------------------------------------------
Values submitted via GET method:
--------------------------------------------------------------------------------
Values submitted via POST method:
variable1 => Value for variable 1
variable2 => Value for variable 2
--------------------------------------------------------------------------------

===> variable0 is missing!!!
So the problem is that, in a POSTed form, variables added to the URL with "?" should be received by the target page as GET variables but the web server is only sending the variables in the form and ignoring the variables in the URL.
I hope this helps.