UTF-8编码怎么去掉BOM头？

<?php
$auto = 1;
checkdir(&#39;C:projectweibo&#39;);
function checkdir($basedir) {
    if ($dh = opendir($basedir)) {
        while (($file = readdir($dh)) !== false) {
            if ($file{0} == &#39;.&#39;) {
                continue;
            }
            if ($file != &#39;.&#39; && $file != &#39;..&#39;) {
                if (!is_dir($basedir . "/" . $file)) {
                    echo "filename: $basedir/$file " . checkBOM("$basedir/$file") . " <br>";
                } else {
                    $dirname = $basedir . "/" . $file;
                    checkdir($dirname);
                }
            }
        }
        closedir($dh);
    }
}
function checkBOM($filename) {
    global $auto;
    $contents = file_get_contents($filename);
    $charset[1] = substr($contents, 0, 1);
    $charset[2] = substr($contents, 1, 1);
    $charset[3] = substr($contents, 2, 1);
    if (ord($charset[1]) == 239 && ord($charset[2]) == 187 && ord($charset[3]) == 191) {
        if ($auto == 1) {
            $rest = substr($contents, 3);
            rewrite($filename, $rest);
            return ("<font color=red>BOM found, automatically removed.</font>");
        } else {
            return ("<font color=red>BOM found.</font>");
        }
    } else return ("BOM Not Found.");
}
function rewrite($filename, $data) {
    $filenum = fopen($filename, "w");
    flock($filenum, LOCK_EX);
    fwrite($filenum, $data);
    fclose($filenum);
}