<?php
    session_start();
    $local = "localhost";
    $login = "root";
    $pass = "root";
    $name = "social";
    $link = mysqli_connect($local,$login,$pass,$name);

    $id = $_SESSION['id'];
    $b = $_GET['id'];
    if(!empty($_POST) && $_POST['mess'] !==""){
        $mess = $_POST['mess'];
        $query = mysqli_query($link,"INSERT INTO message SET from_id='$id',to_id='$b',text_message='$mess'");
        unset($_POST['mess']);
    }
    if(!empty($_GET['id'])){
        $query = mysqli_query($link,"SELECT * FROM message WHERE from_id='$id' AND to_id='$b' OR from_id='$b' AND to_id='$id'");
        while($row = mysqli_fetch_assoc($query)){
            echo $row['text_message']."</br>";
        }
    } else {
    $query = mysqli_query($link,"SELECT * FROM users_friends WHERE user_id='$id'");
    while($row = mysqli_fetch_assoc($query)){
        $friends_id = $row['friends_id'];
        $qu = mysqli_query($link,"SELECT * FROM users WHERE id='$friends_id'");
        $res = mysqli_fetch_assoc($qu);
        echo $res['login']." - <a href='/home/message?id=".$res['id']."'>Написать</a></br>";
    }
    $query = mysqli_query($link,"SELECT * FROM users_friends WHERE friends_id='$id'");
    while($row = mysqli_fetch_assoc($query)){
        $user_id = $row['user_id'];
        $qu = mysqli_query($link,"SELECT * FROM users WHERE id='$user_id'");
        $res = mysqli_fetch_assoc($qu);
        echo $res['login']." - <a href='/home/message?id=".$res['id']."'>Написать</a></br>";
    }}
?>
<?php
    if(!empty($_GET['id'])){
?>
    <form action="" method="POST">
        <textarea name="mess" placeholder="Ваше сообщение"></textarea>
        <input type="submit" value="Send">
    </form>
<?php
    }
?>