PHP-FPM error log says "Not a JPEG file: starts with 0x47 0x49"

Question

In my PHP-FPM log file I have lot's of rows like the following

[18-Sep-2016 03:59:06] WARNING: [pool www] child 5425 said into stderr: "Not a JPEG file: starts with 0x47 0x49"

What does it means?

Answer 1

It is a common error. It looks like you are opening a file with imagecreatefromjpeg, but it is not a jpeg, it is a gif file, take a look at this note concerning starting bytes.

Answer 2

It's most supposely a GIF, as they start with 0x47 0x49. JPG starts with 0xFF 0xD8 0xFF. Have a look at https://en.wikipedia.org/wiki/List_of_file_signatures and reconsider your code: do you want to fail it on such cases (filenames alone are never a guarantee for their content), or do you want to react on it by then guessing its format and trying to parse it as such.