PHP判断上传文件类型

/**

* 读取文件前几个字节 判断文件类型

* @return string

**/
function checkFileType($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;
            break;
    }
    if($strInfo['chars1']=='-1' && $strInfo['chars2']=='-40'){
        return 'jpg';
    }
    if($strInfo['chars1']=='-119' && $strInfo['chars2']=='80'){
        return 'png';
    }
    return $fileType;
}