PHP帮助：在微博中关注系统

I'm making a site similar to Instagram. I am very new to php. I created a follow button in the user's profile.

How do you make the follow button disappear when you already followed the user? How do you replace it with unfollow button?

// my php code for following

if (isset($_POST['addfriend'])){


    $fromuser = $user;
    $touser = $username;

    if($fromuser == $username){
        $Msg = "You cannot follow yourself<br/>";
    }
    else
    {
        $getID= mysql_query("SELECT userID FROM user WHERE username='$user'");
        $get_ID_row = mysql_fetch_assoc($getID);
        $ID_db = $get_ID_row['userID'];

        $sql = "insert into following (userID, fromUser, toUser) 
        values ('$ID_db','$fromuser', '$touser')";

        $result = mysql_query($sql);

         $Msg= "Success! <br/>";

    }
}
else{
    //Do nothing
}

//my code for the follow button

                        <form action="<?php $user;?>" method ="POST">
                        <?php echo $Msg; ?>
                        <input type = "submit" name ="addfriend" value = "Follow"/>
                        </form>

On the page where you are going to show the Follow or Unfollow button, first run a MySQL query to find out if you are already following the person:

$sql = "select * from following 
        where userID = $user
        and   fromUser = $fromUser
        and   toUser = $toUser";
$result = mysql_query($sql);

if( $result) {
    if( mysql_num_rows($result) > 0) {
        // if we get here we know we are already following that person
        ....[see below]

Now dynamically create whichever button you need:-

    if( mysql_num_rows($result) > 0) {
        // if we get here we know we are already following that person
        echo '<input type = "submit" name ="removefriend" value = "Un-follow"/>';
    }
    else
    {
        echo '<input type = "submit" name ="addfriend" value = "Follow"/>';
    }

And on the following page where you are getting the form results, check for both buttons:

if (isset($_POST['addfriend'])) {
    ...[do what you already have]
}
else
if (isset($_POST['removefriend'])) {
    ...[do SQL to remove the record from the following table]
}

Please be aware also that as of PHP v5.5 this style of MySQL is deprecated. At some stage in the future you will have to convert your programs to the MySQLi or PDO_MySQL extensions, before they eventually discontinue support. See the PHP manual about this at eg http://php.net/manual/en/mysqlinfo.api.choosing.php.

Would be easier with OO PHP. However, if you chose procedural, let's assume we have a table of friends. Which keeps the id of each of my friends. e.g.: Smith follows John

Then you do something like $following = mysql_query("SELECT COUNT(*) FROM followers WHERE followerid = ".$_SESSION['id']." AND followeeid = ".$username);

Check if You follow the person already: if($following){//$following == true }