常用类的封装，文件上传类，判断路径是否可用，获取文件信息，检查文件大小，检查文件后缀，移动文件至指定目录的方法，显示错误信息的方法PHP

以下是一个常用的PHP文件上传类的封装，其中包含了判断路径是否可用、获取文件信息、检查文件大小、检查文件后缀、移动文件至指定目录的方法以及显示错误信息的方法：

class FileUploader {
    private $allowedExtensions = array();
    private $sizeLimit = 10485760;
    private $file;

    function __construct(array $allowedExtensions = array(), $sizeLimit = 10485760){
        $allowedExtensions = array_map("strtolower", $allowedExtensions);

        $this->allowedExtensions = $allowedExtensions;
        $this->sizeLimit = $sizeLimit;

        $this->checkServerSettings();

        if (isset($_FILES['file'])) {
            $this->file = new File($_FILES['file']);
        }
    }

    private function checkServerSettings(){
        $postSize = ini_get('post_max_size');
        $unit = strtoupper(substr($postSize, -1));
        $postSize = (int)substr($postSize, 0, -1);
        if ($unit == 'G') {
            $postSize *= 1024 * 1024 * 1024;
        } else if ($unit == 'M') {
            $postSize *= 1024 * 1024;
        } else if ($unit == 'K') {
            $postSize *= 1024;
        }

        if ($postSize < $this->sizeLimit) {
            $this->sizeLimit = $postSize;
        }
    }

    function upload($uploadDirectory){
        if (!is_writable($uploadDirectory)) {
            return "上传目录不存在或不可写";
        }

        if (!$this->file) {
            return "没有文件被上传";
        }

        $size = $this->file->size();

        if ($size == 0) {
            return "文件大小为0";
        }

        if ($size > $this->sizeLimit) {
            return "文件大小超过限制";
        }

        $pathinfo = pathinfo($this->file->name());
        $filename = $pathinfo['filename'];
        $ext = $pathinfo['extension'];

        if ($this->allowedExtensions && !in_array(strtolower($ext), $this->allowedExtensions)) {
            return "文件类型不被允许";
        }

        $filename = md5(uniqid($filename));
        $filename .= '.'.$ext;

        $filePath = $uploadDirectory.'/'.$filename;

        if ($this->file->save($filePath)) {
            return $filename;
        } else {
            return "文件上传失败";
        }
    }
}

class File {
    private $file;

    function __construct($file){
        $this->file = $file;
    }

    function name(){
        return $this->file['name'];
    }

    function size(){
        return $this->file['size'];
    }

    function save($path){
        return move_uploaded_file($this->file['tmp_name'], $path);
    }
}