Cannot connect to any computer except DC

Problem: The only network traffic (ping etc.) that could pass in a Win7/Server 2008 R2 domain was from each client computer to the domain controller. No client computer could contact each other. Even the domain controller could not initiate contact towards a client computer.

Reason: In the Server 2008 R2 active directory domain organizational unit (OU), a Group Policy Object (GPO) had been created to disable the local client's firewall and allow any network traffic (which was ok from a security point of view, since the network was completely disconnected from anything outside). When applied, the firewall was indeed not started, but somehow the idea backfired by blocking all but those client computer connections made to the DC.

Solution: The Firewall service was reenabled in the GPO:
Policies-Windows Settings-Security Settings-System Services-Windows Firewall-Startup type=Automatic
Instead the Firewall setting was set to off for the domain profile (which is the active profile for computers within a domain):
Policies-Windows Settings-Security Settings-Windows Firewall with Advanced Security-Domain Profile Settings-Firewall state = off

It seems absolutely illogical to me that switching off the firewall service would block most network traffic. Maybe some special case in my domain caused it - nevertheless, there was no doubt that my changes were the solution to my problem - even proven by the fact that reversing them would lead to reintroducing the problem.

PHP scripts stopped working when moved

Problem: After having moved some PHP scripts from one web server to another, they stopped working , or only worked partially. One of the webpages showed all the HTML and the upper part of PHP generated code, but strangely after a certain point the rest of the PHP code had just been ignored.

Solution: I loaded one of the script files in the open source DevPHP editor and chose Format on the menu. The Mac (CR) option was checked. I changed it to Windows (CRLF), uploaded the file again and reran it. Voilla, it worked!

Obviously, some PHP parsers may be more picky than others with how they read line breaks.

ODBC 32 vs. 64 bit

Scenario: I needed to connect 32 bit software running on a 64 bit Windows 7 OS to an SQL server via ODBC. Using a User DSN, everything was fine - setting up a System DSN instead, the application could no longer find it or and connect to the database. I preferred setting up a System DSN, so that any user logged on to the computer would be able to run the software without further hassle in setting up the needed ODBC connection.
Cause: An ODBC System DSN in 32 and 64 bit versions are two separate matters, hence a 32-bit application will not see a 64 bit ODBC System DSN and vice versa. User DSNs are however visible on both sides. Confusing.
Gotcha: Both the 64 and 32 bit version of the ODBC tool's is named odbcad32.exe. Hence, the Win7 standard method of writing this in the search field at the bottom of the start menu would always give you the 64 bit version. You have to run %windir%\syswow64\odbcad32.exe (for instance via the run dialog, keyboard shortcut winkey-r) to start the 32 bit ODBC application.
Reference: Microsoft's Knowledge Base article 942976