When $foreign_keys=true in sql_generator.php the resulting SQL for installing Moodle fails in part (i.e. there are other problems beside this one) because several of the foreign key fields do not have the same datatype as the fields they reference.

TABLE `question`, FIELD `category` - should be UNSIGNED
TABLE `portfolio_tempdata`, FIELD `instance` - should be UNSIGNED
TABLE `chat_messages`, FIELD `chatid` - should be UNSIGNED
TABLE `chat_messages_current`, FIELD `chatid` - should be UNSIGNED
TABLE `chat_users`, FIELD `chatid` - should be UNSIGNED and INT(10)
TABLE `scorm_scoes_track`, FIELD `scormid` - should be UNSIGNED