Error calling javascript function in html button

I'm trying to call a JavaScript function on a button in html. Both the button and the JavaScript function are in the same file, however I am having an error saying that the function is not available.

function countFiles() {
  $directory = "/uploadFiles";
  $filecount = 0;
  $files = glob($directory.
    "*");
  if ($files) {
    $filecount = count($files);
  }

  if ($filecount > 0) {
    echo '<script type="text/javascript">',
      'window.location.href = "decrypt.php";',
      '</script>';
  } else {
    echo "<script type='text/javascript'>alert('Precisa de inserir pelo menos 1 ficheiro');</script>";
  }
}

<button onclick="countFiles()">Desencriptar</button>

From your comment,

I want call a function in html button. This function counts the number of files that a directory has. If number of files > 1 the page is redirect.

You can achieve this with an asynchronous call to PHP - through AJAX.

Method 1: Asynchronous call

<script>
function countFiles() {
  $.ajax({
    url: "getFileCount.php", 
    success: function(result) {
        if (result.count > 0) {
            window.location.href = "decrypt.php";
        } else {
            alert('Precisa de inserir pelo menos 1 ficheiro');
        }
    }}
);
</script>

Then, create a file called getFileCount.php

header('Content-Type: application/json');
$directory = "/uploadFiles";
$filecount = 0;
$files = glob($directory . "*");

if ($files){
     $filecount = count($files);
}
echo json_encode(['count' => $filecount]);

Method 2

However, if you don't expect the number of files to be changed between the page load and the click of the button, you don't need AJAX,

<?php 
$directory = "/uploadFiles";
$filecount = 0;
$files = glob($directory . "*");
if ($files){
    $filecount = count($files);
}
?>

<script>
function countFiles() {
    var fileCount = <?php echo $filecount; ?>;
    if(filecount > 0){
       window.location.href = "decrypt.php";
    }else{
        alert('Precisa de inserir pelo menos 1 ficheiro');
    }
}   
</script>