$i = imagecreatefromjpeg("image.jpg");

for ($x=0;$x<imagesx($i);$x++) {
    for ($y=0;$y<imagesy($i);$y++) {
        $rgb = imagecolorat($i,$x,$y);
        $r   = ($rgb >> 16) & 0xFF;
        $g   = ($rgb >>  & 0xFF;
        $b   = $rgb & 0xFF;

        $rTotal += $r;
        $gTotal += $g;
        $bTotal += $b;
        $total++;
    }
}

$rAverage = round($rTotal/$total);
$gAverage = round($gTotal/$total);
$bAverage = round($bTotal/$total);