Assume that $r is 255, and $g and $b are both 0. What is the correct code to output "#ff0000"?
$r
$g
$b
"#ff0000"
printf('#%2X%2X%2X', $r, 0, 0);
printf('#%2x%2x%2x', 255, 0, 0);
printf('#%02x%02x%02x', 255, 0, 0);
printf('#%x%x%x', 255, 0, 0);