Few things could go wrong here.
You need to use POST instead of GET while posting form data.
If data is successfully posted escape text before inserting it into the db with:
mysql_real_escape_string($some_text)
LONGTEXT is just fine for longer text. You don't need LONGBLOB unless you are using binary data (like pictures etc.)
Worst case scenario use some ORM to do the same thing. It will take you some time to configure it to work with your PHP and MySQL but it will save so much time on long run. A good example is Propel.