array.php

$source = ['1:8', '2:8', '1:9', '2:9', '3:1'];
$cleanArray = [];

function split_source($str) {
  $exp = explode(':', $str);
  
  return $exp;
}

foreach ($source as $s) {
  list($question, $answer) = split_source($s);
  
  if (array_key_exists($question, $cleanArray)) {
    array_push($cleanArray[$question], $answer);
  } else {
    $cleanArray[$question] = [$answer];
  }
}

print_r($cleanArray);