Combining 3 databases to display info?

[AidanPT]

Hey, I'm pretty new to this site so I thought I would try it out! You never know!... I am currently building a private messaging system on my website, it's a lot of code so I will try to cut it down, if you require anymore code to help me out, please let me know and I will post it. The problem I am having, in your "inbox" the messages you have sent or received are displayed with the title of (SUBJECT). When you click the subject, you can see all the messages you or the other user have sent to each other. With first and last names. The 2 codes below gather this information from the databases, which is absolutely fine. Although, instead of showing the (SUBJECT) on the inbox page, i would prefer to show the (FIRST NAME + LAST NAME) of who the message is to or from. Just like you can see in the actual message itself. I cannot figure out how to do this. The second code below does this for the view message page, although the top code gathers information for the inbox page, I have tried my best to combine the users info from the 2nd part to the 1st part, but all this does is show every message as the logged in users first and last names, not who the message is sent to or received from, is there a way to do this? Here are the 2 codes:
 

<?php
function fetch_conversation_summery(){
    $sql = "SELECT
                `conversations`.`conversation_id`,
                `conversations`.`conversation_subject`,
                MAX(`conversations_messages`.`message_date`) AS `conversation_last_reply`,
                MAX(`conversations_messages`.`message_date`) > `conversations_members`.`conversation_last_view` AS `conversation_unread`
            FROM `conversations`
            LEFT JOIN `conversations_messages` ON `conversations`.`conversation_id` = `conversations_messages`.`conversation_id`
            INNER JOIN `conversations_members` ON `conversations`.`conversation_id` = `conversations_members`.`conversation_id`
            WHERE `conversations_members`.`userid` = {$_SESSION['userid']}
            AND `conversations_members`.`conversation_deleted` = 0
            GROUP BY `conversations`.`conversation_id`
            ORDER BY `conversation_last_reply` DESC";
            
    $result = mysql_query($sql);
    
    $conversations = array();
    
    while (($row = mysql_fetch_assoc($result)) !== false){
        $conversations[] = array(
            'id'               => $row['conversation_id'],
            'subject'          => $row['conversation_subject'],
            'last_reply'       => $row['conversation_last_reply'],    
            'name'       => $row['first_name'],            
            'unread_messages'  => ($row['conversation_unread'] == 1),

        );
    }
    
    return $conversations;
}

function fetch_conversation_messages($conversation_id){
    $conversation_id = (int)$conversation_id;
    
    $sql = "SELECT
                  `conversations_messages`.`message_date`,
                  `conversations_messages`.`message_date` > `conversations_members`.`conversation_last_view` AS `message_unread`,
                  `conversations_messages`.`message_text`,
                  `users`.`first_name`,
                  `users`.`last_name`
            FROM `conversations_messages`
            INNER JOIN `users` ON `conversations_messages`.`userid` = `users`.`userid`
            INNER JOIN `conversations_members` ON `conversations_messages`.`conversation_id` = `conversations_members`.`conversation_id`
            WHERE `conversations_messages`.`conversation_id` = {$conversation_id}
            AND `conversations_members`.`userid` = {$_SESSION['userid']}
            ORDER BY `conversations_messages`.`message_date` DESC";
            
    $result = mysql_query($sql);
    
    $messages = array();
    
    while (($row = mysql_fetch_assoc($result)) !== false){
        $messages[] = array(
                'date'     => $row['message_date'],
                'unread'   => $row['message_unread'],
                'text'     => $row['message_text'],
                'first_name' => $row['first_name'],
                'last_name' => $row['last_name'],
        );
    }
    
    return $messages;
}

I need to be able to recall the information as "$coversation['first_name']; $conversation['last_name']" to show the names, although this information is only available on the messages page as "$messages['first_name'];. I am currently displaying the subject as "$conversation['subject'];". Trying to turn subject into the first and last names. Files have been attached for reference. Thanks

pmsettings.php

inbox.php

viewmessage.php