This single line would do that:

$array = array_column($array, 'plan');

The first argument is an array | The second argument is an array key.

For details, go to official documentation: https://www.php.net/manual/en/function.array-column.php.

I think you can use array_reduce() function. For example:

   $multi= array(0 => array('t1' => 'test1'),1 => array('t2' => 'test2'),2 => array('t3' => 'test3'),3 => array('t4' => 'test4'));
   $single= array_reduce($multi, 'array_merge', array());
   print_r($single);  //Outputs the reduced aray

I see you want to flatten an array to 1-D. Here is recursive iterator class you can use,

$arr = [["4|1","4|3","4|6"],[["4|1|2","4|1|8"],["4|3|4","4|3|9"],["4|6|5","4|6|12"]]];
$iterator = new \RecursiveIteratorIterator(new \RecursiveArrayIterator($arr));
$result = [];
foreach($iterator as $v) {
  $result[] = $v;
}
print_r($result);

RecursiveArrayIterator - This iterator allows to unset and modify values and keys while iterating over Arrays and Objects in the same way as the ArrayIterator. Additionally, it is possible to iterate over the current iterator entry.

Ref.

Demo

Solution 2:-

$arr = [["4|1","4|3","4|6"],[["4|1|2","4|1|8"],["4|3|4","4|3|9"],["4|6|5","4|6|12"]]];
array_walk_recursive($arr, function($v) use(&$result){
  $result[] = $v;
});
print_r($result);

Demo

Output:-

Array
(
    [0] => 4|1
    [1] => 4|3
    [2] => 4|6
    [3] => 4|1|2
    [4] => 4|1|8
    [5] => 4|3|4
    [6] => 4|3|9
    [7] => 4|6|5
    [8] => 4|6|12
)

array_reduce($array, 'array_merge', array())

Example:

$a = array(array(1, 2, 3), array(4, 5, 6));
$result = array_reduce($a, 'array_merge', array());

Result:

array[1, 2, 3, 4, 5, 6];

PHP have a nice function for this case: array_walk_recursive:

// your multi-dimensional-array
$array = [
    'item1' => 'value1',
    'item2' => 'value2',
    'item3' => [
        'item3a' => 'value3',
    ],
    'item4' => [
        [
            'item4a' => [
                'value4',
                'value5',
            ],
        ]
    ]
];

array_walk_recursive($array, function($a) use (&$new) { $new[] = $a; });
print_r($new);

output

Array
(
    [0] => value1
    [1] => value2
    [2] => value3
    [3] => value4
    [4] => value5
)

Use array_column()

Returns an array of values representing a single column from the input array.

<?php

$user_array = array(
                  0 => array('user_id' => 1, 'name' => 'Bob'),
                  1 => array('user_id' => 2, 'name' => 'John'),
                  2 => array('user_id' => 3, 'name' => 'Mary')
              );

$users = array_column($user_array, 'user_id');

print_r($users);

Output :

Array
(
   [0] => 1
   [1] => 2
   [2] => 3
)

$result = call_user_func_array('array_merge', $array);