Like the Text widget, but also allows working PHP code to be inserted.
That error means that your PHP code is incorrect or otherwise broken.
No, it's not. Really.
This widget has no real errors in it, it's about the simplest widget one can possibly make. Any errors coming out of the "execphp.php" file are errors in code you put into one of the widgets. The reason that it shows the error being in the execphp.php file is because that is where your widget's code is actually being run.
So, if it says that you have an error on line 26, I assure you, the problem is yours. Please don't email me about that error.
Code in a template runs in the global context. Code in the widget will run in a function context. Make sure that you declare any global variables as global before attempting to use them.