<?php
  if(isset($_POST['copyText'])) {
    $text = $_POST['copyText'];
    echo '<textarea id="copyText" style="opacity:0;">'.$text.'</textarea>';
    echo '<script>document.getElementById("copyText").select();document.execCommand("copy");</script>';
  }
?>

<p onclick="copyToClipboard('Bu metin kopyalandı!')">Metni kopyala</p>

<script>
function copyToClipboard(text) {
  var xhr = new XMLHttpRequest();
  xhr.open("POST", "", true);
  xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  xhr.send("copyText=" + encodeURIComponent(text));
}
</script>