PHP image preview upload and generate thumbnail component

The core code of this example comes from the Internet, and the specific source is unknown. This example has been modified and functionally integrated by me.

Key function introduction:

1. Upload images in multiple formats
2. Can generate watermarks
3. You can preview pictures before uploading
4. You can also see the picture immediately after uploading successfully.
5. If you want to re-upload after the upload is completed, the original image will be deleted directly and no junk images will be generated.
6. Thumbnails can be generated, freely set whether you want them or not.

The biggest advantage of this code is to upload pictures. Re-transmitting pictures without saving them to the database will not generate junk pictures. The original pictures will be deleted directly when re-transmitting them.