PHP: Convert RGB to hex and back

Just another note to self, in case I need it again:

/**
 * #rrggbb or #rgb to [r, g, b]
 */
function hex2rgb(string $hex): array
{
    $hex = ltrim($hex, '#');

    if(strlen($hex) == 3)
        return [
            hexdec($hex[0].$hex[0]),
            hexdec($hex[1].$hex[1]),
            hexdec($hex[2].$hex[2]),
        ];
    else
        return [
            hexdec($hex[0].$hex[1]),
            hexdec($hex[2].$hex[3]),
            hexdec($hex[4].$hex[5]),
        ];
}


/**
 * [r, g, b] to #rrggbb
 */
function rgb2hex(array $rgb): string
{
    return sprintf('#%02x%02x%02x', $rgb[0], $rgb[1], $rgb[2]);
}