PHP Manual: Output a PNG image to either the browser or a file

bool imagepng ( resource $image [, string $filename [, int $quality [, int $filters ]]] )

Outputs or saves a PNG image from the given image.

image: An image resource, returned by one of the image creation functions, such as imagecreatetruecolor.
filename: The path to save the file to. If not set or NULL, the raw image stream will be outputted directly.

Note:
NULL is invalid if the quality and filters arguments are not used.
quality: Compression level: from 0 (no compression) to 9.
filters: Allows reducing the PNG file size. It is a bitmask field which may be set to any combination of the PNG_FILTER_XXX constants. PNG_NO_FILTER or PNG_ALL_FILTERS may also be used to respectively disable or activate all filters.

Returns TRUE on success or FALSE on failure.

Version	Description
5.1.3	Added the `filters` parameter.
5.1.2	Added the `quality` parameter.


<?php
$im = imagecreatefrompng("test.png");

header('Content-Type: image/png');

imagepng($im);
imagedestroy($im);
?>

imagepng