Array to JSON conversion resulting empty in PHP [duplicate]

I have a multidimensional array. I want to convert that array to JSON. I'm using json_encode but it's not working.

Here is the code that I'm using.

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

header('Content-Type: application/json');
include '../panel/config/conn.php';
include '../panel/config/helper_functions.php';

$postions = array(1 => "Goalkeeper", 2 => "Defender", 3 => "Midfielder", 4 => "Forward");
$result = array();
$tmp = get_football_assets($conn);

foreach ($tmp as $key => $value) {
    $pair = strtolower($value['pair']);
    $value['pair'] = $pair;
    $value['position'] = $postions[$value['position_id']];
    unset($value['position_id']);
    $result[$pair] = $value;
}
echo json_encode($result);

** Below Function have been written in helper_functions.php **

function get_football_assets($con){
    $data = array();
    $sql = "SELECT nationality, birthdate, birthcountry, birthplace, height, weight, players.position_id as position_id, players.image_path as image_path, teams.name as club, countries.name as country, feed.pair as pair, feed.fullname as name, feed.price as price from players JOIN feed ON feed.player_id = players.api_id JOIN teams ON teams.api_id = players.team_id JOIN countries ON countries.api_id = players.country_id";
    mysqli_set_charset($con, 'utf8');
    $result = mysqli_query($con, $sql);
    while($rows = mysqli_fetch_assoc($result)){
        $data[] = $rows;
    }
    return $data;
}

If I do print_r($result) then it shows the following result but I'm only showing 3 records since the output has more than 1500 records :-

[sergeaurier95094] => Array
        (
            [nationality] => C�te d'Ivoire
            [birthdate] => 24/12/1992
            [birthcountry] => C�te d'Ivoire
            [birthplace] => Ouragahio
            [height] => 176 cm
            [weight] => 76 kg
            [image_path] => https://cdn.sportmonks.com/images/soccer/players/22/95094.png
            [club] => Tottenham Hotspur
            [country] => C�te d'Ivoire
            [pair] => sergeaurier95094
            [name] => Serge Aurier
            [price] => 10.00
            [position] => Defender
        ),

[davinsons�nchezmina26294] => Array
        (
            [nationality] => Colombia
            [birthdate] => 12/06/1996
            [birthcountry] => Colombia
            [birthplace] => Caloto
            [height] => 187 cm
            [weight] => 81 kg
            [image_path] => https://cdn.sportmonks.com/images/soccer/players/22/26294.png
            [club] => Tottenham Hotspur
            [country] => Colombia
            [pair] => davinsons�nchezmina26294
            [name] => Davinson S�nchez Mina
            [price] => 10.00
            [position] => Defender
        ),

[lucasrodriguesmouradasilva95998] => Array
        (
            [nationality] => Brazil
            [birthdate] => 13/08/1992
            [birthcountry] => Brazil
            [birthplace] => S�o Paulo
            [height] => 172 cm
            [weight] => 72 kg
            [image_path] => https://cdn.sportmonks.com/images/soccer/players/30/95998.png
            [club] => Tottenham Hotspur
            [country] => Brazil
            [pair] => lucasrodriguesmouradasilva95998
            [name] => Lucas Rodrigues Moura da Silva
            [price] => 10.00
            [position] => Midfielder
        )

echo json_last_error_msg(); is returning Malformed UTF-8 characters, possibly incorrectly encoded.

Edit array_map I have removed as suggested and I'm using mysqli_set_charset($con, 'utf8'); but it cannot convert characters like alissonrams�sbecker129820, davinsons�nchezmina26294, nabydecoke�ta32640 etc. etc.

echo bin2hex($pair); gives me the following output :-
6875676f6c6c6f72697331303134

6d696368656c766f726d393737

746f6279616c64657277656972656c64393838

73657267656175726965723935303934

62656e646176696573393839

64616e6e79726f736531323731

646176696e736f6e73e3816e6368657a6d696e613236323934

6b696572616e747269707069657231303136

6a616e766572746f6e6768656e393135

6b796c6577616c6b65722d70657465727334363539

62616d6964656c65616c6c69393232

63687269737469616e64616e6e656d616e6e6572696b73656e393932

6572696364696572393136

6572696b6c616d656c61393431

6c75636173726f647269677565736d6f757261646173696c76613935393938

6d6f75737361736973736f6b6f31333830

686172727977696e6b7331373131

68617272796b616e65393937

6665726e616e646f6c6c6f72656e7465746f7272657331363332

6865756e672d6d696e736f6e34333133

6c756b65616d6f733130363830

7061756c6f64696e6f67617a7a616e69676131343131

766963746f7277616e79616d6131313732

6a75616e6d6172636f73666f797468333335373035

616c66696577686974656d616e333430333436

67656f7267656d61727368353430303835

Actually the problem was strtolower. So the updated code which works fine for me is below :-

foreach ($tmp as $key => $value) {
    mb_internal_encoding('UTF-8');
    $pair = mb_strtolower($value['pair']);
    $value['pair'] = $pair;
    $value['position'] = $postions[$value['position_id']];
    unset($value['position_id']);
    $result[$pair] = $value;
}
header('Content-Type: application/json');
echo json_encode($result);