I have optimized the rgb2hsl function from slepichev a bit, so that it is a bit shorter and hopefully a bit faster:

<?php
/**
 * Convert RGB colors array into HSL array
 * 
 * @param array $ RGB colors set, each color component with range 0 to 255
 * @return array HSL set, each color component with range 0 to 1
 */
function rgb2hsl($rgb){
    $clrR = ($rgb[0]);
    $clrG = ($rgb[1]);
    $clrB = ($rgb[2]);
     
    $clrMin = min($clrR, $clrG, $clrB);
    $clrMax = max($clrR, $clrG, $clrB);
    $deltaMax = $clrMax - $clrMin;
     
    $L = ($clrMax + $clrMin) / 510;
     
    if (0 == $deltaMax){
        $H = 0;
        $S = 0;
    }
    else{
        if (0.5 > $L){
            $S = $deltaMax / ($clrMax + $clrMin);
        }
        else{
            $S = $deltaMax / (510 - $clrMax - $clrMin);
        }

        if ($clrMax == $clrR) {
            $H = ($clrG - $clrB) / (6.0 * $deltaMax);
        }
        else if ($clrMax == $clrG) {
            $H = 1/3 + ($clrB - $clrR) / (6.0 * $deltaMax);
        }
        else {
            $H = 2 / 3 + ($clrR - $clrG) / (6.0 * $deltaMax);
        }

        if (0 > $H) $H += 1;
        if (1 < $H) $H -= 1;
    }
    return array($H, $S,$L);
}
?>