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    If all you need is to add the pid to a constant string then you really shouldn't need to modify the database. You can create the string by using something like:
 $extra_info = 'ex' . $pid  
or depending on where you're using this the pid may be available from the object, like:
 $extra_info = 'ex' . $pictureObj->getVar('pid');  
Sorry, I don't know xcgal well enough to tell you which method you're most likely going to be able to use.