<?php
$im = imagecreatefrompng("php.png");
$width = imagesx($im);
$height = imagesy($im);
for ($y = 0; $y < $height; $y++) {
    for ($x = 0; $x < $width; $x++) {
        if (imagecolorat($im, $x , $y) <= 0x7FFFFF) imagesetpixel($im, $x, $y, 0x000000);
        else imagesetpixel($im, $x, $y, 0xFFFFFF);
    } 
} 

header('Content-type: image/png');
imagepng($im);