$_FILES[“file”][“error”]是错误代码，0表示没有错误，下面几种对应不同的错误
1 : 上传的文件超过了 php.ini 中 upload_max_filesize 选项限制的值.
2 : 上传文件的大小超过了 HTML 表单中 MAX_FILE_SIZE 选项指定的值。
3 : 文件只有部分被上传
4 : 没有文件被上传
Error Messages Explained
UPLOAD_ERR_OK
Value: 0; There is no error, the file uploaded with success.
UPLOAD_ERR_INI_SIZE
Value: 1; The uploaded file exceeds the upload_max_filesize directive in php.ini.
UPLOAD_ERR_FORM_SIZE
Value: 2; The uploaded file exceeds the MAX_FILE_SIZE directive that was specified in the HTML form.
UPLOAD_ERR_PARTIAL
Value: 3; The uploaded file was only partially uploaded.
UPLOAD_ERR_NO_FILE
Value: 4; No file was uploaded.
UPLOAD_ERR_NO_TMP_DIR
Value: 6; Missing a temporary folder. Introduced in PHP 4.3.10 and PHP 5.0.3.
UPLOAD_ERR_CANT_WRITE
Value: 7; Failed to write file to disk. Introduced in PHP 5.1.0.
UPLOAD_ERR_EXTENSION
Value: 8; A PHP extension stopped the file upload. PHP does not provide a way to ascertain which extension caused the file upload to stop; examining the list of loaded extensions with phpinfo() may help. Introduced in PHP 5.2.0.