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

In PHP, you can use the "array_reduce" function to flatten a multi-dimensional array. This function takes two arguments: the array to be flattened and a callback function that defines how to combine the array elements. Here is an example of how to flatten a multi-dimensional array by array_reduce function:

<?php

function flattenArray($array)
{
  return array_reduce(
    $array,
    function ($carry, $item) {
      if (is_array($item)) {
        return array_merge($carry, flattenArray($item));
      } else {
        $carry[] = $item;
        return $carry;
      }
    },
    []
  );
}

$multiDimensionalArray = [1, [2, 3], [4, [5, 6]]];
$flattenedArray = flattenArray($multiDimensionalArray);
print_r($flattenedArray);

This will output:

Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 [4] => 5 [5] => 6 )

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Alternatively, you can use call_user_func_array and array_merge recursively to flatten array

<?php

function flatten_array($array)
{
  $flat = [];
  foreach ($array as $value) {
    if (is_array($value)) {
      $flat = array_merge($flat, flatten_array($value));
    } else {
      $flat[] = $value;
    }
  }
  return $flat;
}
$multiDimensionalArray = [1, [2, 3], [4, [5, 6]]];
$flattenedArray = flatten_array($multiDimensionalArray);
print_r($flattenedArray);

This will also output the same result as above.