This is a small script that can be used to calculate moon phases. It's the porting to php of some code lines written in C. I don't remember where I've found the original script, so I can't report the original coder. Sorry. Returning values:

• 0 -> New moon
• 1 -> Waxing crescent moon Right 1–49% visible Left 1–49% visible afternoon and post-dusk 3 pm
• 2 -> First quarter moon Right 50% visible Left 50% visible afternoon and early night 6 pm
• 3 -> Waxing gibbous moon Right 51–99% visible Left 51–99% visible late afternoon and most of night 9 pm
• 4 -> Full moon Fully visible Fully visible sunset to sunrise (all night) 12 midnight
• 5 -> Waning gibbous moon Left 51–99% visible Right 51–99% visible most of night and early morning 3 am
• 6 -> Third quarter moon Left 50% visible Right 50% visible late night and morning 6 am
• 7 -> Waning crescent moon Left 1–49% visible Right 1–49% visible pre-dawn and morning 9 am
• 0 -> Dark moon

function GetMoon ($y, $m, $d)
{
    $c = 0;
    $e = 0;
    $jd = 0.0;
    $b = 0;

    if ($m < 3) {
        $y = $y -1;
        $m = $m + 12;
    }
    $m = $m + 1;
    $c = 365.25 * $y;
    $e = 30.6 * $m;
    $jd = $c + $e + $d-694039.09;  /* jd is total days elapsed */
    $jd /= 29.53; /* divide by the moon cycle (29.53 days) */
    $b = floor(jd); /* int(jd) -> b, take integer part of jd */
    $jd = $jd - $b;	/* subtract integer part to leave fractional part of original jd */
    $b = round($jd * 8); /* scale fraction from 0-8 and round by adding 0.5 */
    $b = $b & 7; /* 0 and 8 are the same so turn 8 into 0 */
    return $b;
}

 Wikipedia: Lunar phase
 Wikipedia: Blue moon

 (ultimo agg. 20/12/12)