Inscribe a circle in a square and paint over with color everything that is outside the circle - is there an easy way?

Question

PHP

Actually, there is a square of arbitrary size.
It is necessary to enter a circle in it with a diameter equal to the side of the square and paint over everything that is outside the circle in black.
Is there a relatively easy way to do this?

Answer 1

1. Draw a solid black square.
   It turned out that the task is to impose a mask on the existing image in the form of a transparent circle in the center and opaque at the edges. D' Normalization gave the correct answer in the comments.