gistfile1.txt

$data = '[{"id":1,"first_name":"Halli","last_name":"Serridge","date":"2017-01-31","location":"Prosyana"},
{"id":2,"first_name":"Henrieta","last_name":"Salzburg","date":"2016-08-20","location":"Antalaha"},
{"id":3,"first_name":"Matteo","last_name":"Fogt","date":"2016-04-15","location":"Fucheng"},
{"id":4,"first_name":"Gray","last_name":"Trosdall","date":"2016-08-15","location":"Yinglong"},
{"id":5,"first_name":"Jerome","last_name":"Edmonson","date":"2017-05-24","location":"San Jose"},
{"id":6,"first_name":"Else","last_name":"Massie","date":"2016-07-14","location":"Tours"},
{"id":7,"first_name":"Marleah","last_name":"Tamblyn","date":"2017-07-29","location":"Kazimierz Dolny"},
{"id":8,"first_name":"Richy","last_name":"Nowaczyk","date":"2017-05-24","location":"Torsby"},
{"id":9,"first_name":"Yuma","last_name":"Torn","date":"2016-10-20","location":"Veruela"},
{"id":10,"first_name":"Leann","last_name":"Giberd","date":"2017-05-31","location":"Najin"}]';

$candidates = json_decode($data, true);

$table = [];
$columnHeaders = ['Weeks'];

foreach ($candidates as $candidate) {
  $date = new \DateTime($candidate['date']);
  if (!isset($table [$date->format('W')])) {
    $table[$date->format('W')] = [];
  }
  if (!isset($table [$date->format('W')][$candidate['location']])) {
    $table[$date->format('W')][$candidate['location']] = 0;
  }
  $table [$date->format('W')][$candidate['location']] += 1;
  if (!in_array($candidate['location'], $columnHeaders)) {
    $columnHeaders[] = $candidate['location'];
  }
}

echo '<table><tr><td>';
echo implode('</td><td>', $columnHeaders);
echo '</td></tr>';

foreach($table as $week => $row) {
  echo '<tr><td>' . $week . '</td>';
  foreach($columnHeaders as $location) {
    echo '<td>';
    echo isset($row[$location])?$row[$location]:0;
    echo '</td>';
  }
  echo '</tr>';
}

//voila
  }
}