JSON将cyrillic编码为null

I have MYSQL table with Cyrillic symbols.
This is my MYSQL table

And i use PHP to get MYSQL result and encode it to JSON.

    <?php

include 'connection.php';

$array_to_json = array();

$query = "SELECT * FROM online";


$result = mysqli_query($link, $query);

mysqli_set_charset("utf8");
while($row = $result->fetch_assoc()) {

$row_array['parameters'] = $row['parameters'];

$row_array['Descriptions'] = $row['Descriptions'];

$row_array['units'] = $row['units'];
      array_push($array_to_json, $row_array);

}

echo json_encode($array_to_json, JSON_UNESCAPED_UNICODE);

$result->close();

?>

And as a result i have got null. JSON returns null

What do i do wrong?

Did you try using

    $row_array['parameters']    = base64_encode($row['parameters']);
    $row_array['Descriptions']  = base64_encode($row['Descriptions']);
    $row_array['units']         = base64_encode($row['units']);
    array_push($array_to_json, $row_array);

recheck your table structure and ensure that there is no extra blank space

for instance:

$row['Descriptions '] 

instead of:

$row['Descriptions'];