Simple script to sync watched episodes and movies from Trakt.tv to any list of IMDb (it hasn't any API available so the calls are made using captured data from a real request in its website) (I know this is against its TOS but the script is for personal use, not commercial).

sync.php

<?php
$traktApiClientId = "TRAKT-APP-CLIENTID"; // You can create an app in https://trakt.tv/oauth/applications

// This fields can be extracted from a real request to add a title to a list
$imdbListId = "list-id";  // list where you want to add watched titles, captured from url called with this format: "https://www.imdb.com/list/<<list-id>>/<<title-id>>/add" (list-id is the needed value)
$imdbCookie = "cookie";   // request header with name "cookie", it must be copied exactly as sent from the website
$imdbPost = "post-data";  // data sent in the body of the request


// TRAKT
$timestampFilename = "time.txt";
$traktApiUrl = "https://api.trakt.tv";

if (file_exists($timestampFilename)) {
    $lastSync = file_get_contents($timestampFilename);
} else {
    $lastSync = 0;
}

// CURL INIT
$ch = curl_init();
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HEADER, false);
curl_setopt($ch, CURLOPT_POST, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    "Content-Type: application/json",
    "trakt-api-version: 2",
    "trakt-api-key: $traktApiClientId",
));

// MOVIES
curl_setopt($ch, CURLOPT_URL, "$traktApiUrl/users/kix2902/watched/movies");
$response = curl_exec($ch);
$data = json_decode($response);

foreach ($data as $item) { // MOVIE
    $updated = strtotime($item->last_updated_at);
    if ($lastSync > $updated) {
        continue;
    }

    $imdbId = $item->movie->ids->imdb;
    if (isset($imdbId)) {
        $imdbItems[] = $imdbId;
    }
}
gc_collect_cycles();
// END_MOVIES

// SHOWS
curl_setopt($ch, CURLOPT_URL, "$traktApiUrl/users/kix2902/watched/shows");
$response = curl_exec($ch);
$data = json_decode($response);

foreach ($data as $item) { // SHOW
    $updated = strtotime($item->last_updated_at);
    if ($lastSync > $updated) {
        continue;
    }

    $showId = $item->show->ids->trakt;
    if (isset($showId)) {
        foreach ($item->seasons as $season) { // SEASON
            $seasonId = $season->number;
            foreach ($season->episodes as $episode) { // EPISODE
                $updated = strtotime($episode->last_watched_at);
                if ($lastSync > $updated) {
                    continue;
                }

                $episodeId = $episode->number;
                curl_setopt($ch, CURLOPT_URL, "$traktApiUrl/shows/$showId/seasons/$seasonId/episodes/$episodeId");
                $epResponse = curl_exec($ch);

                $epData = json_decode($epResponse);

                $imdbId = $epData->ids->imdb;
                if (isset($imdbId)) {
                    $imdbItems[] = $imdbId;
                }
            }
        }
        gc_collect_cycles();
    }
}
file_put_contents($timestampFilename, time());
curl_close($ch);
// END_SHOWS

if (!isset($imdbItems)) {
    return;
}

// IMDB
$error = false;
$ch = curl_init();
foreach ($imdbItems as $item) {
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HEADER, false);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
        "content-type: application/x-www-form-urlencoded",
        "cookie: $imdbCookie",
    ));
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $imdbPost);
    curl_setopt($ch, CURLOPT_URL, "https://www.imdb.com/list/$imdbListId/$item/add");
    $response = curl_exec($ch);

    gc_collect_cycles();
}
curl_close($ch);
// END_IMDB