通过Ajax传递MySQL

I have a simple registration form that uses jQuery and Ajax to validate the user input. When the Ajax tries to do the server side validation of the e-mail address (for example) and connect to the database, it doesn't return a result via $msg.

function checkUser () {
    $search = mysql_query("SELECT * FROM users WHERE username = '" . $username . "'"); 
    $match  = mysql_num_rows($search);

    if($match > 0) {
        $msg .= 'Username already in use, please try another.';
    } 
}

Here is the JS:

function send(datastr) {
    $.ajax({
        type: "POST",
        url: "./scripts/addmember.php",
        dataType: "text",
        data: datastr,
        cache: false,
        success: function(html) {
            $("#errordiv").fadeIn("slow");
            $("#errordiv").html(html);
            //setTimeout('$("#errordiv").fadeOut("slow")',2000);
        }
    });
}

In your checkUser() function you're using $msg and $username, which aren't defined. You either left those out or you are very new to programming.

In PHP, variables usually have only a single "scope". This means that a variable $msg isn't accessible in a function, unless you define it there.

In javascript, this is valid :

var msg = "";
function appendToMsg(text) {
    msg += text;
}
appendToMsg("test");

In PHP, this is not:

$msg = "";
function appendToMsg($text) {
    $msg += $text;
}
appendToMsg("test");

This will throw a notice PHP Notice: Undefined variable: msg. The function appendToMsg will only know $text, not $msg.

The correct PHP representation of the little JavaScript code is:

$msg = "";
function appendToMsg(&$msg, $text) {
    $msg += $text;
}
appendToMsg($msg, "test");