Оп, всъщност не дочетох до край въпроса...

$lines = file("myfile.txt");
$lines_count = count($lines);
$i = -1;
$sum_а = $sum_b = $sum_c = 0;

while (++$i < $lines_count) {
list($ch, $num) = explode(':', $lines[$i]);
if ($ch == 'a') $sum_a += $num;
elseif ($ch == 'b') $sum_b += $num;
elseif ($ch == 'c') $sum_c += $num;
}