streams/include/msglib.php

<?php

/* Common private message processing functions */

function msg_drop($message_id, $channel_id, $conv_guid) {

    // Delete message
    $r = q("DELETE FROM mail WHERE id = %d AND channel_id = %d",
        $message_id,
        $channel_id
    );

    // If it was a first message in thread
    $z = q("SELECT * FROM mail WHERE mid = '%s' AND channel_id = %d",
        $message_id,
        $channel_id
    );
    if (! $z) {
        // Get new first message...
        $r = q("SELECT mid FROM mail WHERE conv_guid = '%s' AND channel_id = %d ORDER BY id ASC LIMIT 1",
            $conv_guid,
            $channel_id
        );
        // ...and refer whole thread to it
        q("UPDATE mail SET parent_mid = '%s', mail_isreply = abs(mail_isreply - 1) WHERE conv_guid = '%s' AND channel_id = %d",
            dbesc($r[0]['mid']),
            $conv_guid,
            $channel_id
        );
    }

}