API YouTube: Escribir comentario en un vídeo

Debes tener PHP 8.0 como mínimo para usar apiClient 2.0
composer require google/apiclient:^2.0

if (!isset($_SESSION)) {

  session_start();

}

DEFINE("GOOGLE_CLIENT_ID","8824XXXX");

DEFINE("GOOGLE_CLIENT_SECRET","GOCSPXXXXX");

DEFINE("GOOGLE_REDIRECT_URL","https://loquesea.com/pruebas/prueba.php");

DEFINE("VIDEO_ID","iouKxxxx");

DEFINE("CHANNEL_ID","UCoM0yxxxxx");

$apiKey = "AIzaSyDwe6war7EzN49EZfxorKXp6Oynvnw8v3g";

$clientId = "882415682099-689roe1l9lm2bjoupekn591n8rib2rc3.apps.googleusercontent.com";

$clientSecret = "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";

$channelId = "UCoM0y3Ys6dE1V-ldTpWKPQQ";

$videoId = "iouKgnDDIX4";

if (!file_exists(__DIR__ . '/vendor/autoload.php')) {

  throw new Exception(sprintf('Please run "composer require google/apiclient:~2.0" in "%s"', __DIR__));

}

require_once __DIR__ . '/vendor/autoload.php';

$client = new Google_Client();

$client->setApplicationName('edc_youtube_add_coment');

$client->setClientId(GOOGLE_CLIENT_ID);

$client->setClientSecret(GOOGLE_CLIENT_SECRET);

$client->setRedirectUri(GOOGLE_REDIRECT_URL);

$client->addScope(['https://www.googleapis.com/auth/youtube.force-ssl']);

if (isset($_GET['code'])) {

    try {

        /*

        $arrAccessToken = $client->authenticate($_GET['code']);

        // print_r($accessToken);exit;

        

        $client->setAccessToken($arrAccessToken["access_token"]);

        */

        

        $arrAccessToken = $client->fetchAccessTokenWithAuthCode($_GET['code']);

        // print_r($arrAccessToken);

        echo "access_token:".$arrAccessToken["access_token"]."<hr>";

        // exit;

        // print_r($arrAccessToken);exit;

        $client->setAccessToken($arrAccessToken);

        // print_r($client);exit;

        $service = new Google_Service_YouTube($client);

        // print_r($service);exit;

        

        // echo "accessTOKEN:";print_r($accessToken);exit;

        $commentSnippet = new Google_Service_YouTube_CommentSnippet();

        $commentSnippet->setTextOriginal("lo que sea dos");

        $commentSnippet->setVideoId(VIDEO_ID);

        $commentSnippet->setChannelId(CHANNEL_ID);

        # Create a top-level comment with snippet.

        $topLevelComment = new Google_Service_YouTube_Comment();

        $topLevelComment->setSnippet($commentSnippet);

        

        # Create a comment thread snippet with channelId and top-level comment.

        $commentThreadSnippet = new Google_Service_YouTube_CommentThreadSnippet();

        $commentThreadSnippet->setVideoId(VIDEO_ID);

        $commentThreadSnippet->setChannelId(CHANNEL_ID);

        $commentThreadSnippet->setTopLevelComment($topLevelComment);

        # Create a comment thread with snippet.

        $commentThread = new Google_Service_YouTube_CommentThread();

        $commentThread->setSnippet($commentThreadSnippet);

        // print_r($commentThread);exit;

        // echo "antes";exit;

        // Call the YouTube Data API's commentThreads.insert method to create a comment.

        

        // print_r($commentThread);

        // echo "a insertar:";

        $resp = $service->commentThreads->insert('snippet', $commentThread);

        // $resp = $service->commentThreads->insert('snippet', $commentThread);

        echo "Todo OK";

    } catch (Exception $exception) {

        print_r($exception->getMessage());

    }    

} else {

    $authUrl = $client->createAuthUrl();

    header('Location: '.$authUrl);

}

---

Otra forma de hacerlo sería mediante JS:

<?php

$apiKey = "AIzaXXXX";

$clientId = "8824XXXX";

$channelId = "UCoM0y3YXCXXXX";

$videoId = "iouKXXXXXX";

?>

<script src="https://apis.google.com/js/api.js"></script>

<script>

  /**

   * Sample JavaScript code for youtube.commentThreads.insert

   * See instructions for running APIs Explorer code samples locally:

   * https://developers.google.com/explorer-help/code-samples#javascript

   */

  function authenticate() {

    return gapi.auth2.getAuthInstance()

        .signIn({scope: "https://www.googleapis.com/auth/youtube.force-ssl"})

        .then(function() { console.log("Sign-in successful"); },

              function(err) { console.error("Error signing in", err); });

  }

  function loadClient() {

    gapi.client.setApiKey("<?=$apiKey;?>");

    return gapi.client.load("https://www.googleapis.com/discovery/v1/apis/youtube/v3/rest")

        .then(function() { console.log("GAPI client loaded for API"); },

              function(err) { console.error("Error loading GAPI client for API", err); });

  }

  // Make sure the client is loaded and sign-in is complete before calling this method.

  function execute() {

    return gapi.client.youtube.commentThreads.insert({

      "part": [

        "snippet"

      ],

      "resource": {

        "snippet": {

          "topLevelComment": {

            "snippet": {

              "textOriginal": "prueba tres",

              "channelId": "<?=$channelId;?>",

              "videoId": "<?=$videoId;?>"

            }

          }

        }

      }

    })

        .then(function(response) {

                // Handle the results here (response.result has the parsed body).

                console.log("Response", response);

              },

              function(err) { console.error("Execute error", err); });

  }

  gapi.load("client:auth2", function() {

    gapi.auth2.init({client_id: "<?=$clientId;?>"});

  });

</script>

<button onclick="authenticate().then(loadClient)">authorize and load</button>

<button onclick="execute()">execute</button>