title: How to set read unread flag in php message system
body: <p>I'm developing a Facebook-type messaging system in PHP. I have a <code>user_message</code> table which contains <code>sender_id</code>, <code>receiver_id</code>, <code>message</code>, and <code>flag</code> as <em>read/unread</em>. After inserting a value in this table, I make it as unread and when the user clicks on the message notification, then the status will be updated as <em>read</em>.</p>
  
  <p>It is working fine for single user. While sending message to multiple users though, I am storing <code>receiver_id</code> as comma separated values in the table. My problem is that how to set flag for multiple receivers? if I'm sending one message to 3 users, for example, then how can I set the <em>read</em> flag? Any help will be Appreciated.</p>