PHP Logic for Username NOT WORKING

Error I'm getting: Whether I try a valid username or password, it always says "Sorry, username already exists" . Is something wrong with my logic, specifically with this code block:

 $q = $conn->query("SELECT uname FROM Student WHERE uname= $aRegUsr ");
    $stmt = $conn->prepare($q);
    $stmt->fetch($q);
    if($stmt > 0){ #if there are 1 or more users with enter username, deny.

dbcred.php

<?php
$host = 'localhost';
$user = 'root';
$pass = '';
$db = 'databasename';

?>

PHP:

<?php
#Login Details
require_once('dbcred.php');
$conn = new PDO("mysql:host=$host;dbname=$db", $user, $pass);

$aRegUsr = $_POS开发者_如何学CT['regduser']; 

#Check for Existing User
$q = $conn->query("SELECT uname FROM Student WHERE uname= $aRegUsr ");
$stmt = $conn->prepare($q);
$stmt->fetch($q);
if($stmt > 0){ #if find that username in use, deny ability to register, else{ALLOW}
echo "Sorry, username already exists";
}
else{
echo "Success";
}

?>

HTML

<form action="inc/check_regUsr.php" method="post" id="userLogon">
    <div class="field required">
        Username: <input type="text" name="regduser" tabindex="1" /><br />
        </div>
        <div class="field required">
        Password: <input type="password" name="regdpass" tabindex="2" /><br />
        </div>
        <input type="submit" name="submitUser" />
</form>

DB Table: Student

Are you trying to create a user registration or user authentication script? I don't believe your actual purpose is clearly outlined to help provide an effective solution.

"Whether I try a valid username or password" indicates user authentication, whereas "if($stmt > 0){ #if there are 1 or more users with enter username, deny." indicates user registration.

EDIT: Based off of your updated requirements, user authentication can get a bit complicated depending on your deployment's security. Included below is sample code based off of your base code. Please note that you may want to find a pre-built script and modify it for your needs as this may serve your purpose better.

<?php #Login Details 
require_once('dbcred.php'); 

#Variables
$error = '';
$auth = 0;
$aRegUsr = $_POST['regduser'];   #Check for Existing User 
$aRegPass = $_POST['regdpass'];   #Check for Existing User 

#DB stuff
$conn = new PDO("mysql:host=$host;dbname=$db", $user, $pass); 
$sql = "SELECT uname,password FROM Student WHERE uname= $aRegUsr ";

#Dataset stuff
foreach($conn->query($sql) as $row){ 
    if($row['password'] != $aRegPass){
        $error = 'Invalid Password';
        $auth = 0; #If $auth = 0 then user is logged in
    }else{
        $auth = 1; #If $auth = 1 then user is logged in
    }
}
$conn = null;
?>

Im not sure as i never used this functionality but from what I understood from here: http://php.net/manual/en/pdostatement.fetch.php fetch return the results so you would actually have to check and see if the count of the results is greater than 0. Check fetchAll http://php.net/manual/en/pdostatement.fetchall.php it should be something like this:

$results = $stmt->fetchAll();
if(count($results)>0)
{
...

Your code is incorrect. I assume your logic in if ($stmt > 0) is to test for if there is already a user existing with that name.

Use this instead -

<?php 

#Check for Existing User
$q = $conn->query("SELECT uname FROM Student WHERE uname= $aRegUsr ");
$stmt = $conn->prepare($q);
$rows = $stmt->fetchAll();
if ($rows > 1) { # The username already exists
    #... rest  of your code