Sql-server – Is it possible to increase the maximum pool size on an ODBC connection

This error likely means that you're not closing your ADO.Net connections. Look through your code, make sure all of your SqlConnection objects are explicitly being properly disposed of in your ASP.Net code. You can do this with using blocks in your VB or C# code. The using construct will automatically close & dispose of your connection when it falls out of scope...

// C# example
using (var conn = new SqlConnection(myConnectString))
{
    conn.Open();

    // your data access code here
}

... or ...

' VB example
Using conn As var = New SqlConnection(myConnectString)

    conn.Open()

    ' your data access code here

End Using

I find that if you look at PHP+ODBC the wrong way when using IIS7 it gives you a generic 500. Getting a 500 error of "0" is extra fun.

I have this feeling you're approaching this from the OS/IIS side when doing it from the PHP code-side may lead us to the fault faster (now that you got your driver issue figured out).

Does PHPInfo work as expected?

Out of the referenced PHP script, what line is it failing at? If you have no other way, just comment out everything inside the PHP tags, see if it works to give you a blank page; then un-comment lines and code sections (inner-most nesting last) one at a time until you find the one that causes the fault to be thrown.

Since your test script is short it'll be easy, and hopfully it'll point out the next thing to check.

HTH.