update a JSON file using PHP/MYSQL - limit size issue

Question

The code below works great until i have around 25000-30000 objects in the Json array stored. Then the file isn't update anymore.
Does someone know what could be the issue?

foreach ($serials as $key => $serial_list) {
    foreach ($serial_list as $key => $serial) {
        $url  = 'json/voltage/voltage_' . $serial . '.json';
        $file = file_get_contents($url, true);
        if ($file != true) {
            $sql    = "SELECT * FROM realtimedata_V WHERE device_serial ='{$serial}' ORDER by timeStamp ASC limit 1200  ";
            $result = $db->mysqli->query($sql);
            while ($row = $result->fetch_assoc()) {
                $voltage_tmp[$serial][] = array(
                    (int) strtotime($row['timeStamp']) * 1000,
                    (int) $row['Va'],
                    (int) $row['Vb'],
                    (int) $row['Vc']
                );

            }
            echo '<hr>';
            echo 'new_datas_' . $serial . '= ' . json_encode($voltage_tmp[$serial]);
            echo '<hr>';
            file_put_contents($url, json_encode($voltage_tmp[$serial]));
        } else {
            $data = json_decode($file, true);
            if ($data === NULL)
                die('Unable to decode');
            unset($file);

         $AllKeys = array_keys($data);
         echo '<hr> Last_Key= <hr>';
          echo $last_index = end($AllKeys);
          echo '<hr>';
         $key = $data[$AllKeys[$last_index]];
         $key_unix = $key[0]/1000;
          $limit_sql = $last_index +2000;
            echo '<hr>';
            echo 'Last_date ' . $serial . '= ' . $last_date = date('Y-m-d H:i:s', $key_unix);
            echo '<hr>';
            echo $sql = "SELECT * FROM realtimedata_V WHERE device_serial ='{$serial}' AND timeStamp > '{$last_date}' ORDER by timeStamp ASC limit {$limit_sql}  ";
            $result  = $db->mysqli->query($sql);
            $row_cnt = $result->num_rows;
            if ($row_cnt === 0) {
                echo 'rows = ' . $row_cnt;
                echo '<hr>Any new datas for the serial N°: ' . $serial . '<hr>';
                continue;
            } else {
                while ($row = $result->fetch_assoc()) {

                    $voltage[$serial][] = array(
                        (int) strtotime($row['timeStamp']) * 1000,
                        (int) $row['Va'],
                        (int) $row['Vb'],
                        (int) $row['Vc']
                    );

                }

                $result = $data + $voltage[$serial];
                echo '<hr>';
                echo 'new_datas_' . $serial . '= ' . $new_data = json_encode($result);
                echo '<hr>';
                if (file_put_contents($url, $new_data) === false) {
                    die('unable to write file');
                }

                unset($result);
                file_put_contents($url, $new_data, LOCK_EX);
                //$result->free();
            }
        }
    }
}

I changed the configuration of the php.ini in case of memory limits. But nothing...
I tried in an another server but same issue. Does someone have an idea?

Thanks