Only allow printable ASCII (UTF8) characters

  <form >
    <input type="text" id="CreatedByText" />
  </form>
  <script>
    document.getElementById('CreatedByText').addEventListener('input', function ()
    {
      //Keep printable ASCII only (space to tilde, 0x20–0x7E):
      let el = document.getElementById('CreatedByText');
      el.value = el.value.replace(/[^\u0020-\u007E]/g, '');
    });
  </script>

Validating in PHP

if(!mb_check_encoding($_POST['MyField'] ?? '', 'UTF-8')) {
  // reject or convert
}