After a few crashes because of no free space left here's a script that notifies you in Slack "errors" channel

slack-space-notofier.php

<?php 

define('THRESHOLD', 80);

$res = shell_exec('/bin/df -h | /bin/grep \'/dev/sda2\' | /usr/bin/awk \'{print $5}\'');
$res = floatval(str_replace('%', '', trim($res))); 
if($res > THRESHOLD){
  $ch = curl_init();
  curl_setopt($ch, CURLOPT_URL, 'https://hooks.slack.com/services/T01234567/B11234567/123456712345671234567');
  curl_setopt( $ch, CURLOPT_POSTFIELDS, json_encode([
      'channel' => 'errors',
      'username' => 'Space Commando',
      'icon_emoji' => ':rage3:',
      'text' => '<@alex>: LOW SPACE on tb03: *' . $res . '%*'
  ]));
  curl_setopt( $ch, CURLOPT_HTTPHEADER, array('Content-Type:application/json'));
  curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );
  $result = curl_exec($ch);
  curl_close($ch);
}