<?
function hex2rgbs(string $colour, float $alpha = 1) {
	if (!empty($colour)) {
		if($alpha >= 0.95) {
			return $colour;
		}
		else {
			if($colour[0] == '#') {
				$colour = substr($colour, 1);
			}
			if(strlen($colour) == 6) {
				list($r, $g, $b) = array(
					$colour[0] . $colour[1],
					$colour[2] . $colour[3],
					$colour[4] . $colour[5]
				);
			}
			elseif (strlen($colour) == 3) {
				list($r, $g, $b) = array(
					$colour[0] . $colour[0],
					$colour[1] . $colour[1],
					$colour[2] . $colour[2]
				);
			}
			else {
				return false;
			}
			$r = hexdec($r);
			$g = hexdec($g);
			$b = hexdec($b);
			$output = array(
				'red' => $r,
				'green' => $g,
				'blue' => $b
			);
			return 'rgba(' . implode($output, ',') . ',' . $alpha . ')';
		}
	}
}
$s = "{color hex=\"fff\" alpha=\"0.8\"} {color hex=\"fff\"}";
$s = preg_replace_callback("#\{color hex=\"(\w+)\"\s?(alpha=\"([0-9.]+)\")?\}#is", function($matches) {
	return hex2rgbs($matches[1], ($matches[3]) ? $matches[3] : 1);
}, $s);
echo $s;