PHP Manual: imagepng

function imagepng(
    GdImage $image,
    resource|string|null $file = null,
    int $quality = -1,
    int $filters = -1
): bool

Outputs or saves a PNG image from the given image.

image: A GdImage object, returned by one of the image creation functions, such as imagecreatetruecolor().
file: The path or an open stream resource (which is automatically closed after this function returns) to save the file to. If not set or null, the raw image stream will be output directly.

Note:
null is invalid if the quality and filters arguments are not used.
quality: Compression level: from 0 (no compression) to 9. The default (-1) uses the zlib compression default. 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_* constants. PNG_NO_FILTER or PNG_ALL_FILTERS may also be used to respectively disable or activate all filters. The default value (-1) disables filtering.

Caution
The filters parameter only has an effect when PHP uses the bundled libgd. It is ignored when PHP is built against a system libgd. Check GD_BUNDLED or gd_info() to see which one is in use.

Returns true on success or false on failure.

Caution

However, if libgd fails to output the image, this function returns true.

Throws a ValueError if quality is invalid.

Version	Description
8.4.0	Now throws a ValueError if `quality` is invalid.
8.0.0	`image` expects a GdImage instance now; previously, a valid `gd` resource was expected.

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

header('Content-Type: image/png');

imagepng($im);
?>

imagepng