which files may read by a php?

Question

this question is asked for the purpose that the file is created by php is only allowed to read or other files can be allowed to read by php

Answer 1

Good ! that's is a cool question..
basically all files are stored as 1s and 0s in memory . It's basically the contents represented differ ..
for example
A text file contains set of ASCII value
image files also contains 1s and 0s. but there it represents the color value of pixels on the screen

it's not about which file
it's all about the file function that you use to read or write
example
fopen() fread() fwrite() are functions in PHP to manage basic text file
json_encode() used for reading json files
etc..