<?php         
                                                                                      
$server_key = 'AAAAnCw-yKA:APA91bEYphFbq_w...';

$title = 'Title';
$text = 'test';
$limit = 999;
$field_name = 'DeviceToken';

   $sql = mysqli_connect("mysqlserver.com", "DBName", "DBPassword");
   /* check connection */
   if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    exit();
}
   
   $offset = 0;
   while (true) {  
                                                           
   $query = "SELECT $field_name FROM `DBName`.`TableName` LIMIT $limit OFFSET $offset";
   $result = mysqli_query($sql, $query);                 
   if (!$result) {
    die('Invalid query: ' . mysql_error());
} 
   if (mysqli_num_rows($result) == 0) {
     echo "{\"result\":true}";  
     exit;    
   }
  $arr = array();  
  while ($row = mysqli_fetch_array($result, MYSQL_ASSOC)) {
    $arr[] =  $row["$field_name"];  
   }
   
  pushSend($title, $text, $arr, $server_key); 


  $offset = $offset + $limit;
  /* free result set */
  mysqli_free_result($result);  


 //  foreach($arr as $item) {
 //    echo $item, '<br>';
//}

//echo '-----------<br>';

}

mysqli_close($sql);
         
       

// max 1000
 
function pushSend($title, $text, $tokens, $server_key) {
    $url = 'https://fcm.googleapis.com/fcm/send';
    $headers = array('Authorization: key=' . $server_key, 
     'Content-Type: application/json');
  
    if (is_array($tokens))
      $fields['registration_ids'] = $tokens;
    else
      $fields['registration_ids'] = array($tokens);
  
    $fields['priority'] = 'high';
    $fields['notification'] = array('body' => $text, 'title' => $title);
    $fields['data'] = array('message' => $text, 'title' => $title);
  
    $ch = curl_init();
    curl_setopt_array($ch, array(
            CURLOPT_URL => $url,
            CURLOPT_POST => true,
            CURLOPT_HTTPHEADER => $headers,
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_SSL_VERIFYHOST => 0,
            CURLOPT_SSL_VERIFYPEER => false,
            CURLOPT_POSTFIELDS => json_encode($fields)
    ));
    $result = curl_exec($ch);
    curl_close($ch);
    return $result;
}
?>

function pushSend($title, $text, $tokens, $server_key) {
    $url = 'https://fcm.googleapis.com/fcm/send';
    $headers = array('Authorization: key=' . $server_key, 
     'Content-Type: application/json');
  
    if (is_array($tokens))
      $fields['registration_ids'] = $tokens;
    else
      $fields['registration_ids'] = array($tokens);
  
    $fields['priority'] = 'high';
    $fields['notification'] = array('body' => $text, 'title' => $title);
    $fields['data'] = array('message' => $text, 'title' => $title);
  
    $ch = curl_init();
    curl_setopt_array($ch, array(
            CURLOPT_URL => $url,
            CURLOPT_POST => true,
            CURLOPT_HTTPHEADER => $headers,
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_SSL_VERIFYHOST => 0,
            CURLOPT_SSL_VERIFYPEER => false,
            CURLOPT_POSTFIELDS => json_encode($fields)
    ));
    $result = curl_exec($ch);
    curl_close($ch);
    return $result;
}