php - imagepng, set quality without writing to file, how? -

i want export png memory:

imagepng ($img, ?, 9); 

i want compress, need set filename too. dont want it, how then?

you this:

<?php ob_start(); imagepng($img, null, 9, png_all_filters); $image_data = ob_get_contents(); ob_end_clean(); 

from manual:

filename: path save file to. if not set or null, raw image stream outputted directly.