Hey,
sorry for taking so long to reply. Kind of a busy week around here.
You can visit the IMS BasicLTI site
http://www.imsglobal.org/developers/BLTI/ if you have not already done so. Apart from documentation, you can find a zip file containing the source code of some simple examples. Look into tool.php for the code of a very simple tool provider. The actual validation code is around line 137 of blti.php
The zip file also contains the OAuth.php file we use to sign all messages.
Any more questions, don't hesitate to contact again.
Nikolas