php twitter: undefined index: oauth_token_secret

The problem is that there is an error in the request so you don't get oauth_token_secret in the respond from Twitter. Instead you get an error message.

Turn of php notice by typing the following at the top of the page
error_reporting(E_ALL ^ E_NOTICE);

print_r() the $request_token or $request variable to the screen. It will show you the type of error returned by twitter
print_r($request_token );

An example of the error message when you print the array is "Desktop applications only support the oauth_callback value 'oob' /oauth/request_token?oauth_callback=...."

Go to your app settings on twitter and make the correct settings.
In the case of the above error, set the callback url to the one you send in the request and try again!!

Good Luck.