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 value) { 
    if (is_array($value)) { 
      $result = array_merge($result, array_flatten($value)); 
    } 
    else { 
      $result[value; 
    } 
  } 
  return $result; 
} 

You can use as follows :

$newArray = array();
foreach($arrayData as value) {
    foreach($value as $key2 => $value2) {
        $newArray[$key2] = $value2;
    }
}

Where $arrayData is your DB data array and $newArray will be the result.

You can use array_merge().

$arrays = [["4|1","4|3","4|6"],[["4|1|2","4|1|8"],["4|3|4","4|3|9"],["4|6|5","4|6|12"]]]
$arrayMerge = []
foreach($arrays as $array)
{
    $arrayMerge = array_merge($arrayMerge, $array)
}

For more info: https://www.php.net/manual/es/function.array-merge.php

You can use recusice function to convert multidimensional arrays to single array.

function filter($array)
{
    static $newArray;
    if (is_array($array)):
        array_map('filter', $array);
    else:
        $newArray[] = $array;
    endif;

    return $newArray;
}


$array = [
    'isim' => 'Şahin',
    'soyisim' => 'ERSEVER',
    'yabanci_dil' => [
        'tr' => 'Türkçe',
    ],
    'languages' => [
        [
            'php' => [
                'codeigniter',
                'laravel',
                'symfony'
            ],
            'javascript' => [
                'vuejs',
                'react' => [
                    'react',
                    'react-native'
                ]
            ]
        ]
    ]
];


print_r(filter($array));

Result:

Array
(
    [0] => Şahin
    [1] => ERSEVER
    [2] => Türkçe
    [3] => codeigniter
    [4] => laravel
    [5] => symfony
    [6] => vuejs
    [7] => react
    [8] => react-native
)

The PHP array_merge^­Docs function can flatten your array:

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

In case the original array has a higher depth than 2 levels, the SPL in PHP has a RecursiveArrayIterator you can use to flatten it:

$flat = iterator_to_array(new RecursiveIteratorIterator(new RecursiveArrayIterator($array)), 0);

See as well: How to Flatten a Multidimensional Array?.

Where $array is the multidimensional array you provided above:

$data = array();
foreach ($array as $item) {
    $data[] = $item['user_id'];
}
print_r($data);

Like this:

$result = array();

foreach($array as $inner) {
    $result[key($inner)] = current($inner);        
}

The $result array would now look like this:

Array
(
    [Price] => 1
    [Size] => 7
    [Type] => 2
)