Convert multidimensional array into single array

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.

Assuming this array may or may not be redundantly nested and you're unsure of how deep it goes, this should flatten it for you:

function array_flatten($array) { 
  if (!is_array($array)) { 
    return FALSE; 
  } 
  $result = array(); 
  foreach ($array as $key => $value) { 
    if (is_array($value)) { 
      $result = array_merge($result, array_flatten($value)); 
    } 
    else { 
      $result[$key] = $value; 
    } 
  } 
  return $result; 
} 

If you come across a multidimensional array that is pure data, like this one below, then you can use a single call to array_merge() to do the job via reflection:

$arrayMult = [ ['a','b'] , ['c', 'd'] ];
$arraySingle = call_user_func_array('array_merge', $arrayMult);
// $arraySingle is now = ['a','b', 'c', 'd'];

Just assign it to it's own first element:

$array = $array[0];

For this particular case, this'll do:

$array = array_map('current', $array[0]);

It's basically the exact same question is this one, look at some answers there: PHP array merge from unknown number of parameters.