php判断文件类型magic bytes

<?php
/**
 * 读取文件前几个字节 判断文件类型
 *
 * @return String
 */
function checkTitle($filename) {
    $file = fopen($filename, "rb");
    $bin = fread($file, 2); //只读2字节
    fclose($file);
    $strInfo = @unpack("c2chars", $bin);
    $typeCode = intval($strInfo['chars1'] . $strInfo['chars2']);
    $fileType = '';
    switch ($typeCode) {
        case 7790:
            $fileType = 'exe';
            break;

        case 7784:
            $fileType = 'midi';
            break;

        case 8297:
            $fileType = 'rar';
            break;

        case 255216:
            $fileType = 'jpg';
            break;

        case 7173:
            $fileType = 'gif';
            break;

        case 6677:
            $fileType = 'bmp';
            break;

        case 13780:
            $fileType = 'png';
            break;

        default:
            $fileType = 'unknown' . $typeCode;
    }
    //Fix
    if ($strInfo['chars1'] == '-1' && $strInfo['chars2'] == '-40') {
        return 'jpg';
    }
    if ($strInfo['chars1'] == '-119' && $strInfo['chars2'] == '80') {
        return 'png';
    }
    return $fileType;
}
?>