<?php
// Tableau représentant un jeu d'enregistrements issu d'une base de données

$records = array (
  0 => 
  array (
    'id' => '3',
    'groupname' => 'test',
    'cinema_group' => '2',
    'cinema_name' => 'test_1',
    'cinema_location' => 'test_location1',
  ),
  1 => 
  array (
    'id' => '4',
    'groupname' => 'test',
    'cinema_group' => '2',
    'cinema_name' => 'test_2',
    'cinema_location' => 'test_location2',
  ),
  2 => 
  array (
    'id' => '5',
    'groupname' => 'test',
    'cinema_group' => '2',
    'cinema_name' => 'test3',
    'cinema_location' => 'test_location3',
  ),
  3 => 
  array (
    'id' => '6',
    'groupname' => 'test',
    'cinema_group' => '2',
    'cinema_name' => 'test4',
    'cinema_location' => 'test_location4',
  ),
);

$first_names = array_column($records, null, 'id');

print_r($first_names);