<script>
  var insta = "<?php echo $instapost; ?>";
  function addinsta() {
    document.getElementById("txtpost").innerHTML = insta;
  }
</script>

<button id="instabutton" onclick="addinsta()">Generate Post</button><br />
<textarea id="txtpost" name="txtpost"></textarea>

<script>
  var insta = <?php echo json_encode($instapost); ?>;

  function addinsta() {
    document.getElementById("txtpost").value = insta;
  }
</script>

<script>
  var insta = `<?php echo $instapost; ?>`;
  function addinsta() {
    document.getElementById("txtpost").innerHTML = insta;
  };
</script>