How to "flatten" a multi-dimensional array to simple one in PHP?

This is a one line, SUPER easy to use:

$result = array();
array_walk_recursive($original_array,function($v) use (&$result){ $result[] = $v; });

It is very easy to understand, inside the anonymous function/closure. $v is the value of your $original_array.

If you specifically have an array of arrays that doesn't go further than one level deep (a use case I find common) you can get away with array_merge and the splat operator.

<?php

$notFlat = [[1,2],[3,4]];
$flat = array_merge(...$notFlat);
var_dump($flat);

Output:

array(4) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
  [3]=>
  int(4)
}

The splat operator effectively changes the array of arrays to a list of arrays as arguments for array_merge.

$array  = your array

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

echo "<pre>";
print_r($result);

REF: http://php.net/manual/en/function.call-user-func-array.php

Here is another solution (works with multi-dimensional array) :

function array_flatten($array) {

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

}

$array  = Your array

$result = array_flatten($array);

echo "<pre>";
print_r($result);

Use array_walk_recursive

<?php

$aNonFlat = array(
    1,
    2,
    array(
        3,
        4,
        5,
        array(
            6,
            7
        ),
        8,
        9,
    ),
    10,
    11
);

$objTmp = (object) array('aFlat' => array());

array_walk_recursive($aNonFlat, create_function('&$v, $k, &$t', '$t->aFlat[] = $v;'), $objTmp);

var_dump($objTmp->aFlat);

/*
array(11) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
  [3]=>
  int(4)
  [4]=>
  int(5)
  [5]=>
  int(6)
  [6]=>
  int(7)
  [7]=>
  int(8)
  [8]=>
  int(9)
  [9]=>
  int(10)
  [10]=>
  int(11)
}
*/

?>

Tested with PHP 5.5.9-1ubuntu4.24 (cli) (built: Mar 16 2018 12:32:06)