php 判斷 hex color 然後回傳 白色 or 黑色

getContrastColor.php

    /**
     * @param $hexColor
     * @return string
     */
    function getContrastColor($hexColor)
    {
        $R1 = hexdec(substr($hexColor, 1, 2));
        $G1 = hexdec(substr($hexColor, 3, 2));
        $B1 = hexdec(substr($hexColor, 5, 2));

        $blackColor = "#000000";
        $R2BlackColor = hexdec(substr($blackColor, 1, 2));
        $G2BlackColor = hexdec(substr($blackColor, 3, 2));
        $B2BlackColor = hexdec(substr($blackColor, 5, 2));

        $L1 = 0.2126 * pow($R1 / 255, 2.2) +
            0.7152 * pow($G1 / 255, 2.2) +
            0.0722 * pow($B1 / 255, 2.2);

        $L2 = 0.2126 * pow($R2BlackColor / 255, 2.2) +
            0.7152 * pow($G2BlackColor / 255, 2.2) +
            0.0722 * pow($B2BlackColor / 255, 2.2);

        $contrastRatio = ($L1 > $L2)
            ? (int)(($L1 + 0.05) / ($L2 + 0.05))
            : (int)(($L2 + 0.05) / ($L1 + 0.05));

        return ($contrastRatio > 5)
            ? '#000000'
            : '#FFFFFF';
    }