index.html

<style>
  .error {
    color: red;
  }
</style>

<form>
  <label for="t1">t1:</label>
  <input type="text" id="t1">
  <button type="button" id="save">zapisz</button>
</form>
<pre id="response"></pre>

<script>
  window.onload = load;

  function load() {
    const button_save = document.querySelector('#save'),
          tag_response = document.querySelector('#response');

    button_save.onclick = save;

    function save(e) {
      e.preventDefault();

      const t1 = document.querySelector('#t1').value || '';
      button_save.disabled = true;

      const form_data = new FormData();
      form_data.append('t1', t1);

      postData(form_data);
    }

    async function postData(form_data) {
      try {
        const response = await fetch('t1_example.php', {
          method: 'POST',
          body: form_data
        });

        const data = await response.text();
        if (data) {
          tag_response.innerHTML = data;
        } else {
          throw 'Brak danych z serwera';
        }
      } catch(err) {
        console.warn('postData:\n' + err);
        tag_response.innerHTML = `<span class="error">${err}</span>`;
      } finally {
        setTimeout(() => {
          button_save.disabled = false;
        }, 500);
      }
    }
  }
</script>