Rows with a variable number of columns?
Edited after a couple of comments by the question asker!:
Better idea for your design: 
(If you're not familar with these diagrams, that's seven tables, with foreign key links as pictured [the three _Link tables all have two foreign keys referencing the data tables]. It probably looks like I'm overcomplicating things, but trust me, &quot;tokenise once and store in the database&quot; is so much more efficient than &quot;tokenise data each time you retrieve it&quot;.)
Sample data, using a simple tweet (ignoring Users for now, it's the same concept though): &quot;test tweet! #howdoesthiswork #newbie&quot;
      Tweet
TweetID   WordCount
   1         2

Tweet_Word_Link
TweetID   WordID
   1         1
   1         2

      Word
WordID    Word
   1      test
   2      tweet

Tweet_Hashtag_Link
TweetID  HashtagID
   1         1
   1         2

      Hashtag
HashtagID  Hashtag
    1      #howdoesthiswork
    2      #newbie

This allows you to easily get all the users linked in a tweet:
SELECT U.UserName
FROM  User U
        INNER JOIN
      Tweet_User_Link TUL ON U.UserID = TUL.UserID
        INNER JOIN
      Tweet T ON TUL.TweetID = T.TweetID
WHERE T.TweetID = &lt;your ID here&gt;
      

(change table names as appropriate for Hashtags and Words)
Or, slightly more complicated, get the top 10 words for all tweets mentioning a specific hashtag:
SELECT TOP 10 W.Word, COUNT(*) AS WordCount
FROM  Word W
        INNER JOIN
      Tweet_Word_Link TWL ON TWL.WordID = W.WordID
        INNER JOIN
      Tweet T ON TWL.TweetID = T.TweetID
        INNER JOIN
      Tweet_Hashtag_Link THL ON THL.TweetID = T.TweetID
        INNER JOIN
      Hashtag H ON H.HashtagID = THL.HashtagID
WHERE H.Hashtag = &lt;your hashtag here&gt;
GROUP By W.Word

(I've used ANSI SQL syntax, I'm not 100% sure if MySQL supports all of ANSI SQL - but it should be close enough.)