How can I tell if a URL points to an image?

Question

I have a php page with a text input where the user is supposed to paste a remote URL of an image, and I will have to store it in the server and display it to the user. Now the problem is, I don't trust a user will always provide a proper image url, and I don't want them to upload a pdf or other file, or a huge, few gb worth of file. Now I can check the extension, but that isn't very helpful, and I hear I can check the mime-type, but I don't know how I can open the file once and check all the validations like mime-type and file size in one go, and then copy the file over. Moreover, since the file will be pretty much served as it is(with a minor name change), I would like to know if it is possible to make sure that the file doesn't have any injected virus or problematic code.

Any suggestions appreciated.

Answer 1

You can use exif_imagetype() to see if its an image.

If you want to be 100% sure that its not malware or something weird. its a good idea to use the GD library and save it via the GD library. So there is no dangerous code inside.