EvilCatRu

/((\d+?)\.(\d+?)),/.exec('17629,17629,"ПОНТОННЫЙ, ВОКЗАЛ",59.785067,30.627792,0,2,bus')

$data = [
    1 => 'a',
    'b',
    'c',
    'd',
    'e',
    'f',
    'g',
    'h',
];

$day = 9;

if ($day > ($maxDay = max(array_keys($data)))) {
    $day = $day % $maxDay;
}

echo $data[$day];