php怎么将二维数组转为多个一维数组

$user = array(
    '0' => array('id' => 100, 'username' => 'a1'),
    '1' => array('id' => 101, 'username' => 'a2'),
    '2' => array('id' => 102, 'username' => 'a3'),
    '3' => array('id' => 103, 'username' => 'a4'),
    '4' => array('id' => 104, 'username' => 'a5'),
);

$result = array(100, 'a1'， 101, 'a2', 102, 'a3', 103, 'a4', 104, 'a5');

$result = array_reduce($user, function ($result, $value) {
    return array_merge($result, array_values($value));
}, array())

$user = array(
    'a' => array(100, 'a1'),
    'b' => array(101, 'a2'),
    'c' => array(102, 'a3'),
    'd' => array(103, 'a4'),
    'e' => array(104, 'a5'),
);

$result = array_reduce($user, 'array_merge', array())

$result = [];
array_walk_recursive($user, function($value) use (&$result) {
    array_push($result, $value);
});

$user4 = array(
    'a' => array(100, 'a1'),
    'b' => array(101, 'a2'),
    'c' => array(
        'd' => array(102, 'a3'),
        'e' => array(103, 'a4'),
    ),
);

$result = array(100, 'a1'， 101, 'a2', 102, 'a3', 103, 'a4');

$result = [];
array_map(function ($value) use (&$result) {
    $result = array_merge($result, array_values($value));
}, $user);