There is an error I experienced in PHP Designer, but it has been experienced with other software too.

After some fiddling, here is the fix.

The problem originates from PHP Designer's built in PHP engine. It's the standard PHP which can be downloaded from here PHP.net .
The problem file is libmysql.dll.
It's located at C:\program files\PHP Designer\PHP\

Simply Download PHP 5.2+ from PHP.net It's the Windows Binary version you need, other packages don't have this file.
Unzip and replace the libmysql.dll from the PHP Designer's PHP directory, with the libmysql.dll from the PHP package you just downloaded.

When you start PHP Designer and attempt to execute PHP code, you won't see the problem again.