This is a PHP function to check if a file has a UTF8 encoding, UTF8 files that only contain ASCII characters are detected as us-ascii by PHP.

gistfile1.php

<?php
 
function isUTF8($filename)
{
    $info = finfo_open(FILEINFO_MIME_ENCODING);
    $type = finfo_buffer($info, file_get_contents($filename));
    finfo_close($info);
 
    return ($type == 'utf-8' || $type == 'us-ascii') ? true : false;
}