PHP function to convert HEX color code to RGB

This little function has proved quite handy in converting (configurable) HEX colour codes to RGB colour codes ready for usage in CSS rgba() colour declarations.

function hex2rgb($hex) {
    $hex = str_replace("#", "", $hex);
    if (preg_match("/^([a-f0-9]{3}|[a-f0-9]{6})$/i",$hex)):      // check if input string is a valid hex colour code
        if(strlen($hex) == 3) { // three letters code
           $r = hexdec(substr($hex,0,1).substr($hex,0,1));
           $g = hexdec(substr($hex,1,1).substr($hex,1,1));
           $b = hexdec(substr($hex,2,1).substr($hex,2,1));
        } else { // six letters coode
           $r = hexdec(substr($hex,0,2));
           $g = hexdec(substr($hex,2,2));
           $b = hexdec(substr($hex,4,2));
        }
        return implode(",", array($r, $g, $b));         // returns the rgb values separated by commas, ready for usage in a rgba( rr,gg,bb,aa ) CSS rule
        // return array($r, $g, $b); // alternatively, return the code as an array
    else: return "";  // input string is not a valid hex color code - return a blank value; this can be changed to return a default colour code for example
    endif;
} // hex2rgb()