PHP错误-创建的所有文件或目录均为0

Question

I was recently coding a registration script and wanted to create a folder inside another folder called "users". Somehow, the folder created was in the root path (where the php script is located) and the files, that were supposed to be written inside the folder were in a file called 0. Here's the code:

if (!isset($_POST["method"])){
                die("Error");
            }
            if (!isset($_POST["usernamefld"])){
                die("Error");
            }
            if (!isset($_POST["passwordfld"])){
                die("Error");
            }
            if ($_POST["method"] == "register"){
                if (!isset($_POST["emailfld"])){
                    die("Error");
                }

                if(is_dir("./users/"+$_POST["usernamefld"])){
                    die("Taken");
                }

                mkdir("/users/"+$_POST["usernamefld"]);
                echo "test";



            }

The Folder "0" is always empty when created.

Answer 1

As @u_mulder has stated, in PHP we concat strings and variables with a "." and not the "+" symbol. Based on that, this should help with your particular issue.

if (!isset($_POST["method"])){
    die("Error");
}
if (!isset($_POST["usernamefld"])){
    die("Error");
}
if (!isset($_POST["passwordfld"])){
    die("Error");
}

if ($_POST["method"] == "register"){
    if (!isset($_POST["emailfld"])){
        die("Error");
    }

    if(is_dir("./users/". $_POST["usernamefld"])){
        die("Taken");
    }

    mkdir("/users/". $_POST["usernamefld"]);
    echo "test";
 }