PHP group array by date with interval

It is not clearly described which time differences should be used. The solution here always calculates the time differences to the highest date of the group.

The array will be sorted in descending order of the date. The highest date is set as $groupDate. A new $result array is created with a foreach loop. If the date difference to $groupDate is less than $intervalHour, then 'content' is accumulated. In the other case a new $groupDate is set. At the end the result array is sorted again according to increasing date.

$data = [
  ['date' => '2020-10-07 10:49:48', 'content' => 1],
  ['date' => '2020-10-08 13:49:48', 'content' => 2],
  ['date' => '2020-10-08 13:50:03', 'content' => 3],
  ['date' => '2020-10-08 14:50:04', 'content' => 4],
];

$intervalHour = 1.0;

usort($data,function($a,$b){return $b['date'] <=> $a['date'];});

$groupDate = date_create($data[0]['date']);
$content = $data[0]['content'];
$result = [$data[0]];
$k = 0;
foreach($data as $i => $row){
  if($i == 0) continue;
  $diff = date_create($row['date'])->diff($groupDate);
  $diffHour = $diff->days * 24 + $diff->h + $diff->i/60 + $diff->s/3600;  
  if($diffHour < $intervalHour) {
    $content = $row['content'].','.$content;
  }
  else {
    ++$k;
    $groupDate = date_create($row['date']);
    $result[$k]['date'] = $groupDate->format('Y-m-d H:i:s');
    $content = $row['content'];
  }
  $result[$k]['content'] = $content;
}

usort($result,function($a,$b){return $a['date'] <=> $b['date'];});

echo '<pre>',var_export($result);

Output with $intervalHour = 1.0

array (
  0 => 
  array (
    'date' => '2020-10-07 10:49:48',
    'content' => 1,
  ),
  1 => 
  array (
    'date' => '2020-10-08 13:50:03',
    'content' => '2,3',
  ),
  2 => 
  array (
    'date' => '2020-10-08 14:50:04',
    'content' => 4,
  ),
) 

Output with $intervalHour = 2.0

array (
  0 => 
  array (
    'date' => '2020-10-07 10:49:48',
    'content' => 1,
  ),
  1 => 
  array (
    'date' => '2020-10-08 14:50:04',
    'content' => '2,3,4',
  ),
) 

The problem is that your unset command does not work. When you use a foreach on an array, a copy of that array will be created and iterated. If you want to iterate over an instance that you can change on the fly, you would need to pass an iterable object.

As an example, the following would output 12, despite the unset:

$x = [1, 2];

foreach ($x as $key => $value) {
    echo $value;
    unset($x[1]);
}

I think best approach is using OOP. But if you want to work only with forloops and arrays, it could be done like this:

  $usedKeys = [];
  $dateContent  =[];

  foreach ($result as $key => $value) {
      if (in_array($key, $usedKeys)) {
          continue;
      }
      $usedKeys[] = $key;
      $dateContent[$key]['content'] = [$value['content']];
      foreach ($result as $key2 => $value2) {
          if (in_array($key2, $usedKeys)) {
              continue;
          }

          if (abs(strtotime($value['date']) - strtotime($value2['date'])) <= 3600 && $key != $key2) {
              $dateContent[$key]['content'][] = $value2['content'];
              $usedKeys[] = $key2;
          }
      }
  }

  $dateContent = array_map(function ($value) {
      return implode(',', $value['content']);
  }, $dateContent);