$_POST[]在iOS客户端中丢失了符号字符串

我的客户端通过Post发送到"abc+def"，然后再次从服务器获取字符串
但是返回字符串是"abcdef"。为什么？如何保持abc+def？

-iOS客户端-

NSError * error=nil;
NSString * str=[NSString stringWithFormat:@"selected_card=@"abc+def"];
NSData * postData= [str dataUsingEncoding:NSUTF8StringEncoding];
NSString *myString = [[NSString alloc] initWithData:postData encoding:NSUTF8StringEncoding];
NSLog(@"myString=%@",myString);
[req setHTTPMethod:@"POST"];
[req setValue:@"application/json" forHTTPHeaderField:@"Accept"];
[req setHTTPBody:postData];
NSHTTPURLResponse * response=nil;

NSData *data=[NSURLConnection sendSynchronousRequest:req returningResponse:&response error:&error];
if(data!=nil)
{
    NSLog(@"error:%@",error);
    NSDictionary * cardDictionary= [NSJSONSerialization JSONObjectWithData:data
                                                                   options:NSJSONReadingMutableLeaves
                                                                     error:&error];
    NSLog(@"return string:%@",[cardDictionary objectForKey:@"card1"]);
}

-服务器

...
$user_table = $_POST['id']."_card";
$select_card_drawing_url=$_POST['selected_card'];
$response=array("card1" => $select_card_drawing_url);
echo json_encode($response);
return ;
?>

-客户端日志-

MyString=abc+def
return String: abc def