Need to change pdo fetching mode

Using PHP PDO and psql
My query:

$stmt = $this->pdo->prepare("select distinct extract(year from conclusion) from agreements_register where conclusion is not null");
$stmt->execute();
return $stmt->fetchAll();

it returns this:

[['date_part' => 2021],['date_part' => 2022]]

but I need this:

[2021, 2022]

>Solution :

this is example:

<?php
$sth = $dbh->prepare("SELECT name, colour FROM fruit");
$sth->execute();

$result = $sth->fetchAll(PDO::FETCH_COLUMN, 0);
var_dump($result);
?>

and it will output

Array(3)
(
    [0] =>
    string(5) => apple
    [1] =>
    string(4) => pear
    [2] =>
    string(10) => watermelon
)

and detail you can read this doc:
https://www.php.net/manual/en/pdostatement.fetchall.php

i hope it works for you