Tutorial Create Data URI’s
These can be useful for embedding images into HTML/CSS/JS to save on HTTP requests, at the cost of maintainability. More information. There are online tools to do it, but if you want your own very simple utility, here's some PHP to do it:
function data_uri($file, $mime) {
$contents=file_get_contents($file);
$base64=base64_encode($contents);
echo "data:$mime;base64,$base64";
}