Generate a marker with a given background and foreground colour.

marker.php

<?php

/* .htaccess
RewriteRule markers?/([0-9a-fA-F]{6})/([0-9a-fA-F]{6})/([^/]+)/marker.png markers/index.php?bg=$1&fg=$2&icon=$3
*/
header('Content-type: image/png');

// read parameters: icon file, foreground and background colors
$bgc = sscanf(empty($_GET['bg']) ? 'FFFFFF' : $_GET['bg'], '%2x%2x%2x');
$fgc = sscanf(empty($_GET['fg']) ? '000000' : $_GET['fg'], '%2x%2x%2x');
$icon = empty($_GET['icon']) ? 'base.png' : $_GET['icon'] . '.png';

// read image information from template files
$shadow = imagecreatefrompng("../static/img/marker/shadow.png");
$bg = imagecreatefrompng("../static/img/marker/bg.png");
$fg = imagecreatefrompng("../static/img/marker/" . $icon);
$base = imagecreatefrompng("../static/img/marker/base.png");
imagesavealpha($base, true); // for the "shadow"

// loop over every pixel
for($x=0; $x<imagesx($base); $x++) {
        for($y=0; $y<imagesy($base); $y++) {
                $color = imagecolorsforindex($bg, imagecolorat($bg, $x, $y));
                // templates are grayscale, any channel serves as alpha
                $alpha = ($color['red'] >> 1) ^ 127; // 127=transparent, 0=opaque.
                if($alpha != 127) { // if not 100% transparent
                        imagesetpixel($base, $x, $y, imagecolorallocatealpha($base, $bgc[0], $bgc[1], $bgc[2], $alpha));
                }
                // repeat for foreground and shadow with foreground color
                foreach(array($shadow, $fg) as $im) {
                        $color = imagecolorsforindex($im, imagecolorat($im, $x, $y));
                        $alpha = ($color['red'] >> 1) ^ 127;
                        if($alpha != 127) {
                                imagesetpixel($base, $x, $y, imagecolorallocatealpha($base, $fgc[0], $fgc[1], $fgc[2], $alpha));
                        }
                }
        }
}
// spit image
imagepng($base);
// destroy resources
foreach(array($shadow, $fg, $base, $bg) as $im) {
        imagedestroy($im);
}

?>