How to increase image size without scale image with php

I want to change .webp image width or height to a highter value, and add some transparent area to image.

For example, imagine I have an width:512*height:312 webp image,I need two steps:

1- Add 512*200 transparent area to my image.

2- Transform my image to the center of new image.

OR

1- Add 512*100 transparent area to the top of my image.

2- Add 512*100 transparent area to the bottom of my image.

Final result i want, an 512*512 .webp image without any scale on my original image.

How can I do this with php language?