How to convert a png image to a jpg image using the GD Library

How can I convert a png image to a jpg image using the GD library? The basic requirement is: reduce the size of the png image, then maintain a certain definition, and the size remains unchanged. How to implement it? How can I convert a png image to a jpg image using the GD library?
The basic requirement is: reduce the size of the png image, then maintain a certain definition, and the size remains unchanged. How to implement it? Share images:
------ Solution --------------------
$ Im = imagecreatefrompng ('source Image title ');
Imagejpeg ($ im, 'New Image title ');