Color.php

<?php

class Color
{
    /**
     * Convert a hex color to RGB.
     * 
     * @param  string $hex        #BADA55
     * @return array{int,int,int} [186, 218, 85]
     */
    public static function rgb(string $hex): array
    {
        return sscanf($hex, "#%02x%02x%02x");
    }

    /**
     * Calculate the relative luminance of an RGB color.
     * 
     * @param  int $r
     * @param  int $g
     * @param  int $b
     * @return float
     */
    public static function luminance(int $r, int $g, int $b): float
    {
        return 0.2126 * pow($r/255, 2.2) +
               0.7152 * pow($g/255, 2.2) +
               0.0722 * pow($b/255, 2.2);
    }

    /**
     * Calculate the relative luminance of two colors.
     * 
     * @param  string $C1 hex color
     * @param  string $C2 hex color
     * @return float
     */
    public static function relativeLuminance(string $C1, string $C2): float
    {
        $L1 = self::luminance(...self::rgb($C1));
        $L2 = self::luminance(...self::rgb($C2));

        if ($L1 > $L2) {
            return ($L1 + 0.05) / ($L2 + 0.05);
        } else {
            return ($L2 + 0.05) / ($L1 + 0.05);
        }
    }
}

// Check if white text against #bada55 background passes G18.
// https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20160317/G18
return Color::relativeLuminance('#ffffff', '#bada55') >= 4.5;

Hi, I think there is an issue, a contrast ratio between #ffffff and #bada55 is 1.58:1 https://webaim.org/resources/contrastchecker/?fcolor=ffffff&bcolor=BADA55

This is correct:

return Color::relativeLuminance('#000000', '#bada55') >= 4.5; // Return true