I've changed a little bit your code. Hope it will helpfull:
#New DB connection
@ $mysqli = new mysqli ('localhost', '…', '…', '…');
#Check DB connection, print error upon failure
if (mysqli_connect_errno()) {
printf("<p>Connect failed.</p> <p>Error: \"%s\"</p>\n", mysqli_connect_error());
exit();
} #if
#Check if userName already exists
$query = "SELECT * FROM wp_users WHERE user_login = ?";
$stmt = $mysqli->prepare($query);
$stmt->bind_param("s", $_GET[registerUserName]);
$stmt->execute();
$stmt->store_result();
$num_results = $stmt->num_rows;
if ($num_results) {
echo "<p>This user already exists. Please click your back button.</p>";
exit;
}; #if
Now we can call it with URL like "/ttt.php?registerUserName=user1" And function:
function checkForUser(uname)
{
var req;
req = null;
if (window.XMLHttpRequest) {
try {
req = new XMLHttpRequest();
} catch (e){}
} else if (window.ActiveXObject) {
try {
req = new ActiveXObject('Msxml2.XMLHTTP');
} catch (e){
try {
req = new ActiveXObject('Microsoft.XMLHTTP');
} catch (e){}
}
}
if (req) {
req.open("GET", "/ttt.php?registerUserName="+encodeURIComponent(uname), false);
//req.onreadystatechange = processReqChange;
req.send(null);
if(req.status == 200) {
return req.responseText;
}
}
return false;
}
in the
req.open("GET", "/ttt.php?registerUserName="+encodeURIComponent(uname), false);
false means async request and req.send will wait for response from php script.