Javascript textbox value into PHP code?

Name: <input type="text" id="myText" value="Mickey">

<button onclick="myFunction()">Try it</button>

<script>
function myFunction() {
  document.getElementById("myText").value = "Johnny Bravo";
}
</script>

   <?php
$myfile = fopen("newfile.txt", "w") or die("Unable to open file!");
$txt = "myText\n";
fwrite($myfile, $txt);
$txt = "Jane Doe\n";
fwrite($myfile, $txt);
fclose($myfile);
?>

$txt = *******?;