Formula for decoding px from size?

Question

PHP

Let's just assume that a file came from the server.
I don't know in advance which one.
After a series of checks, I know for sure that this is a
valid png picture of size 90170
How to get PXxPX from this value It is
possible to find out the ratio before uploading the picture to the server.

Answer 1

You need to at least read the PNG header (or rather IHDR chunk , which comes first), it specifies the width and height in pixels. Without access to the contents of the file, this cannot be determined.