function conversion($color, $param) {
    $r = substr($color, 1, 2);
    $g = substr($color, 3, 2);
    $b = substr($color, 5, 2);
    $r = hexdec($r);
    $g = hexdec($g);
    $b = hexdec($b);
    if ($newR >= $param) {$newR = $r - $param;} else if ($newR <= 255) {$newR = $r + $param;}
    if ($newG >= $param) {$newG = $g - $param;} else if ($newG <= 255) {$newG = $g + $param;}
    if ($newB >= $param) {$newB = $b - $param;} else if ($newB <= 255) {$newB = $b + $param;}
    $newR = dechex($newR);
    $newG = dechex($newG);
    $newB = dechex($newB);
    return "#" . $newR . $newG . $newB;
}

echo conversion('#0073aa', 10); //resultado #a7db4