PHP验证是否为图片格式文件

/**
 * 判断是否为图片格式(jpg/jpeg/gif/png)文件
 *
 * @param string $filePath
 * @return bool|string
 */
function isImg($filePath)
{
    if (!is_file($filePath)) {
        return false;
    }
    $file = fopen($filePath, "rb");
    $bin = fread($file, 2); // 只读2字节    fclose($file);
    $strInfo = @unpack("C2chars", $bin);
    $typeCode = intval($strInfo['chars1'] . $strInfo['chars2']);
    switch ($typeCode) {
        case 255216:
            $fileType = 'jpg';
            break;
        case 7173:
            $fileType = 'gif';
            break;
        case 13780:
            $fileType = '13780';
            break;
        default:
            $fileType = false;
            break;
    }
    return $fileType;
}