Group SQL query results by a field

try this:

SELECT id_foreign,group_concat(concat(' . ',cosa) separator '<br>') as cosas    
from table
group by id_foreign

I do not know if you use PDO , mysqli , etc, so I'll do it in a separate code:

<?php
/* Simulo una variable con tu contenido para obtener registros de ella
  basta con un SELECT * FROM tabla , no hay necesidad de usar GROUP BY */
$registros = [
  [ 'ID' => 1, 'id_foreign' => 1, 'cosa' => 'cosa1' ],
  [ 'ID' => 2, 'id_foreign' => 1, 'cosa' => 'cosa2' ],
  [ 'ID' => 3, 'id_foreign' => 2, 'cosa' => 'cosa1' ],
  [ 'ID' => 4, 'id_foreign' => 3, 'cosa' => 'cosa1' ],
  [ 'ID' => 5, 'id_foreign' => 4, 'cosa' => 'cosa1' ],
  [ 'ID' => 6, 'id_foreign' => 4, 'cosa' => 'cosa2' ],
  [ 'ID' => 7, 'id_foreign' => 4, 'cosa' => 'cosa3' ],
  [ 'ID' => 8, 'id_foreign' => 4, 'cosa' => 'cosa4' ],
];

$datos = [];
/* Creamos una matriz con índice 'id_foreign' con elementos variables */
foreach ($registros as $registro) {
  $datos[$registro['id_foreign']][] = $registro;
}
?><table border="1">
<tr><th>id_foreign</th><th>cosa</th></tr>
<?php
/* Obtenemos una a una cada una de las id_foreign */
foreach ($datos as $id_foreign => $fila) {
  /* Ahora, por cada elemento que tenga cada id_foreign creamos una fila */
  foreach ($fila as $indice => $elemento) {
?>  <tr>
<?php
    /* Si es el primer elemento deberemos crear la celda que muestra el id_foreign y decirle el alto (la cantidad de elementos que contiene) */
    if ($indice === 0) {
?>    <td rowspan="<?= count($fila) ?>"><?= htmlentities($id_foreign) ?></td>
<?php
    }
?>    <td><?= htmlentities($elemento['cosa']) ?></td>
  </tr>
<?php
  }
} 
?>
</table>

If you need to group fields, I think you should look at the GROUP BY clause Example link