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

bool imagepng ( resource $image [, mixed $to [, 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.
to: The path or an open stream resource (which is automatically being closed after this function returns) 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. The current default is 6. For more information see the » zlib manual.
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
5.4.0	Added support for passing a stream resource to `to`.
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