hsv2rgb()

這個函式的用途是將以 HSV 表示的顏色轉換成 RGB 色碼，演算法來自 Wikipedia ，目前網站的隨日期變色效果就是用此 function 來達成的。

$H 的範圍為 0–360 , $S 的範圍為 0–100 , $V 的範圍為 0–100

function hsv2rgb($H, $S, $V)
{
    if ( $V == 0 )
    {
        $R = 0;
        $G = 0;
        $B = 0;
    }
    else if ( $S == 0 )
    {
        $V = $V / 100;
        $R = $V * 255;
        $G = $V * 255;
        $B = $V * 255;
    }
    else
    {
        //這一段是讓 $H 也能輸入小於 0 或大於 360 的數字，這樣用起來比較方便
        if ( $H > 360 )
        {
            $H = $H % 360;
        }
        else if ( $H < 0 )
        {
            $H = 360 + ( $H % 360 );
        }

        $S = $S / 100;
        $V = $V / 100;
        $Hf = $H / 60;
        $Hi = floor( $Hf );
        $f = $Hf - $Hi;
        $p = $V * ( 1 - $S );
        $q = $V * ( 1 - $S * $f );
        $t = $V * ( 1 - $S * ( 1 - $f ) );

        switch ( $Hi ) {
            case 0:
                $R = $V * 255;
                $G = $t * 255;
                $B = $p * 255;
                break;
            case 1:
                $R = $q * 255;
                $G = $V * 255;
                $B = $p * 255;
                break;
            case 2:
                $R = $p * 255;
                $G = $V * 255;
                $B = $t * 255;
                break;
            case 3:
                $R = $p * 255;
                $G = $q * 255;
                $B = $V * 255;
                break;
            case 4:
                $R = $t * 255;
                $G = $p * 255;
                $B = $V * 255;
                break;
            case 5:
                $R = $V * 255;
                $G = $p * 255;
                $B = $q * 255;
                break;
        }
    }

    return '#' . str_pad(dechex(round($R)), 2, "0", STR_PAD_LEFT) . str_pad(dechex(round($G)), 2, "0", STR_PAD_LEFT) . str_pad(dechex(round($B)), 2, "0", STR_PAD_LEFT);
}