php判断一个文件编码

<?php
define ('UTF32_BIG_ENDIAN_BOM'   , chr(0x00) . chr(0x00) . chr(0xFE) . chr(0xFF));
define ('UTF32_LITTLE_ENDIAN_BOM', chr(0xFF) . chr(0xFE) . chr(0x00) . chr(0x00));
define ('UTF16_BIG_ENDIAN_BOM'   , chr(0xFE) . chr(0xFF));
define ('UTF16_LITTLE_ENDIAN_BOM', chr(0xFF) . chr(0xFE));
define ('UTF8_BOM'               , chr(0xEF) . chr(0xBB) . chr(0xBF));
//discuz core
$_req = trim($_GET['url']);
header("Content-Type: text/plain;charset=utf-8");


if( 'UTF-16LE' === detect_utf_encoding( $_req)){
    echo iconv('UTF-16', 'UTF-8', file_get_contents( $_req));
}else {
    echo file_get_contents( $_req);
}

function detect_utf_encoding($filename) {

    $text = file_get_contents($filename);
    $first2 = substr($text, 0, 2);
    $first3 = substr($text, 0, 3);
    $first4 = substr($text, 0, 3);

    if ($first3 == UTF8_BOM)
        return 'UTF-8';
    elseif ($first4 == UTF32_BIG_ENDIAN_BOM)
        return 'UTF-32BE';
    elseif ($first4 == UTF32_LITTLE_ENDIAN_BOM)
        return 'UTF-32LE';
    elseif ($first2 == UTF16_BIG_ENDIAN_BOM)
        return 'UTF-16BE';
    elseif ($first2 == UTF16_LITTLE_ENDIAN_BOM)
        return 'UTF-16LE';
}

?>