php - How to handle the server error correctly?

I face a big problem and would like you to help me solve one.

While uploading files on server I receive the "POST Content-Length of 323344106 bytes exceeds the limit of 5242880 bytes in Unknown on line 0" error.
I know a meaning of the error, but I want to catch the error and show it to users; namely, I wanna show the error, for example, below the HTML form in order that users can understand why to receive the error.