发送http发布请求时出错

我使用此方法向服务器发送post请求并将数据保存到sql数据库但我遇到了一个问题，当数据保存到数据库时，我漏掉了一些字符例如，我有一个字符串"examplepost++request++"当接收到数据库字符串时，由于缺少+加号而更改"示例发布请求"这是我的代码

xcode

NSString *STR = @"mystring=example post ++ request ++";
NSData *PostData = [STR dataUsingEncoding:NSUTF8StringEncoding];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:@"http://MySiteUrl.com/example.php"]];
    [request setValue:@"gzip" forHTTPHeaderField:@"Accept-Encoding"];
    [request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
    [request setHTTPBody:PostData];
    [request setHTTPMethod:@"POST"];
    NSError *error = nil;
    NSURLResponse *response = nil;
    NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];

php代码

$STR = $_POST['mystring'];
          header ('Content-Type: text/html; charset=UTF-8'); 
$con    = mysql_connect("localhost","xxxxxx","xxxx");
          mysql_select_db("xxxxxxx", $con);
          mysql_query("SET NAMES utf8");
$STR = trim($STR);
 mysql_query("INSERT INTO `xxxxxxx`.`table` (`id`, `mystring`) VALUES (NULL, '$STR');");