Is there a simpler way to achieve this style of user messaging?

Question

I have created a messaging system for users, it allows them to send a message to another user. If it is the first time they have spoken then a new conversation is initiated,

Answer 1

If you're asking for a way to be able to keep all of your current functionality and work flows, yet keep the data in a single table I think you're pretty close.

Instead of having the conversationId be a key to a different table, I would instead have it point to the ID of the message that began the conversation. This would create a parent-child relationship between messages that began a conversation and all those that followed after it. To be able to see all conversations, you would just select all messages where the conversationId is null. Below is a representation of a 2 message conversation:

+----+---------+------+------------------+----------------+--------+----------+
| id | message | read | time             | conversationId | toUser | fromUser |
+----+---------+------+------------------+----------------+--------+----------+
| 1  |  test 1 |  0   | (some timestamp) |  null          |  3     |   4      |
| 2  |  test 2 |  0   | (some timestamp) |   1            |  4     |   3      |
+----+---------+------+------------------+----------------+--------+----------+

The conversation was initiated by user 3. All messages in the conversation can be filter by conversationId. One limitation of this design is that only 2 users can be apart of the conversation.

Update

You could get the last message given a conversation id this way:

SELECT id, message 
FROM userMessages 
WHERE conversationId = {conversationId} 
ORDER BY time DESC 
LIMIT 1