Deleting specific data from a text document using fwrite?

Question

I am writing an "unfriend" script for my mini social network, basically the unfriend action needs to remove their id called: $user from their friend list and my id called: $my_id. I have made the scripts that add user id's to each users files but I am not sure how to delete this?

My friend_list file: 1,2,3 // I am user 1

Their friend_list file: 1,2,3 // They are user 3

I currently use this to add each id into the text file:

if($action === 'accept'){
    mysql_query("DELETE FROM `friend_req` WHERE `from`='$user' AND `to`='$my_id'");

    $fp = fopen($user_data['friend_list'], 'a');
    fwrite($fp, ",".$user."");
    fclose($fp);

    $their_id = friend_list_from_user_id($user);
    $fp = fopen($their_id, 'a');
    fwrite($fp, ",".$my_id."");
    fclose($fp);
}

But to remove $my_id from their friend_list and their $userid from my friend_list I'm thinking I need to use something like this but it isn't working:

if($action === 'unfriend'){
    $fp = fopen($user_data['friend_list'], 'a');
    unset($user);
    fclose($fp);

    $their_id = friend_list_from_user_id($user);
    unset($my_id);
    fclose($fp);
}

But that doesn't seem to work, what should I do to delete only the specified usernames from the respective file?

Oh, and also, this may/may not be of use:

$myFile = $file;
$fh = fopen($myFile, 'r');
$theData = fgets($fh);
fclose($fh);

$friend = explode(',', $theData);

for($i=0; $i < count($friend); $i++){
    if($i >= 0)
    {
        if($friend[$i] == $user_id) {
        $their_user_id = $user_id;
    }
}

Answer 1

To solve your issue just do something like:

//get the contents of the file
$contents = file_get_contents($user_data['friend_list']);
$contents = str_replace(",".$friend_id, "", $contents); //remove the friend's id

//open the file again and rewrite the whole thing with the new contents without that id
$fp = fopen($user_data['friend_list'], 'w+');
fwrite($fp, $contents);
fclose($fp);