Google Apps Script to post filtered Gmail messages to Slack

gmailToSlack.js

/*
The intent of this script is for posting filtered Gmail messages to Slack. 
This script could be used on its own with manually-marked messages, but it
is most useful it when combined with a Gmail filter. The script assumes that
target messages have had a specific label set on them and have been starred.
The Apps Script can then be set to run periodically.

2015/02 cmyers, rush


The MIT License (MIT)

Copyright (c) 2015 DF Studio

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

function checkForMessages() {
  
  var slackChannel = 'THE_SLACK_CHANNEL';
  var webhookURL = 'THE_WEBHOOK_URL';
  var slackUsername = 'THE_USERNAME';
  
  var label = GmailApp.getUserLabelByName('THE_LABEL_ASSIGNED_WITH_THE_FILTER');
  var threads = label.getThreads();
  
  for (var t = 0; t < threads.length; t++){
    var thread = threads[t];
    if (thread.hasStarredMessages()){
      var messages = thread.getMessages();
      for (var m = 0; m < messages.length; m++){
        var message = messages[m];
        if (message.isStarred()){
          postToSlack(webhookURL, slackChannel, slackUsername, message.getPlainBody());
          message.unstar();
        }
      }
    }
  }
}

function postToSlack(url, channel, username, text) {
   var payload = {
     'channel': channel,
     'username': username,
     'text': text
   };
   
   var payloadJson = JSON.stringify(payload);
  
   var options = {
     'method': 'post',
     'contentType': 'json',
     'payload': payloadJson
   };

   UrlFetchApp.fetch(url, options);
}

Great script 👍
If needed PHP alternative here