CentOS 7.3, PostgreSQL 9.6.2, PHP 5.4 deliver array as string

Good evening!

I am calling this stored function -

CREATE OR REPLACE FUNCTION words_get_board(
in_gid integer
) RETURNS TABLE (
out_bid integer,
out_letters varchar[15][15],
out_values integer[15][15]
) AS
$func$
SELECT
bid,
letters,
values
FROM words_games
WHERE gid = in_gid;

$func$ LANGUAGE sql STABLE;

by a PHP script -

$sth = $dbh->prepare(SQL_GET_BOARD);
$sth->execute(array($gid));
if ($row = $sth->fetch(PDO::FETCH_ASSOC)) {
$bid = $row['bid'];
$letters = $row['letters'];
$values = $row['values'];
}

And then print the variable type -

error_log('letters: ' . gettype($letters));

and it is a "string" (instead of inspected array) with the following
content:

[02-Mar-2017 21:28:33 Europe/Berlin] letters: string
[02-Mar-2017 21:28:33 Europe/Berlin] letters:
{{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,П,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,К,А,Й,Т,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,П,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,Ь,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,Е,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL},{NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL}}

How to handle this please?

I was hoping to fetch a PHP array and process it with "foreach" in my
PHP-script.

Thank you
Alex