PHP mysql search multiple tables using a keyword

I have three tables in my database which are:

messages
topics
comments

Each of these tables has two fields called 'content' and 'title'. I want to be able to use 'Like' in my sql statement to look at 'messages.content', 'messages.title', 'topics.content', 'topics.title', 'comments.content' and 'comments.title' using a keyword.

So far, my query is able to find results from only one table:

mysql_query("SELECT * FROM messages 
WHERE content LIKE '%" . $keyword . "%' 
OR title LIKE '%" . $keyword ."%'");

I am also wondering, once I get the results from multiple tables, how can I tell what result is from what table?

Any help would be greatly appreciated!

$query = "(SELECT content, title, 'msg' as type FROM messages WHERE content LIKE '%" . 
           $keyword . "%' OR title LIKE '%" . $keyword ."%') 
           UNION
           (SELECT content, title, 'topic' as type FROM topics WHERE content LIKE '%" . 
           $keyword . "%' OR title LIKE '%" . $keyword ."%') 
           UNION
           (SELECT content, title, 'comment' as type FROM comments WHERE content LIKE '%" . 
           $keyword . "%' OR title LIKE '%" . $keyword ."%')";

mysql_query($query);

So, you are getting result from all of the three tables, and you can identify which row came from which table by looking at its type value.

What you are probably looking for is the UNION command:

SELECT id, 'messages' as 'table' FROM messages 
  WHERE content LIKE '%keyword%' 
    OR title LIKE '%keyword%'
UNION
SELECT id, 'topics' as 'table' FROM topics
  WHERE content LIKE '%keyword%' 
    OR title LIKE '%keyword%'
UNION
SELECT id, 'comments' as 'table' FROM comments
  WHERE content LIKE '%keyword%' 
    OR title LIKE '%keyword%'