MySQL query not executing properly - php

Question

I want to make a PHP login script, that when user signs in, it removes the Sign-In form with another div saying "Welcome [user_name]". I am running the script on same page as my html, but the query always fails. Can anyone please sort out this problem, why is this happening?
PHP CODE:
<?php include("connect.php")?>
<?php
session_start();
//Function to sanitize values received from the form. Prevents SQL injection
function clean($str) {
$str = #trim($str);
if(get_magic_quotes_gpc()) {
$str = stripslashes($str);
}
return mysql_real_escape_string($str);
}
if(isset($_POST['username']) && isset($_POST['username'])){
//Sanitize the POST values
$UserName = clean($_POST['username']);
$Password =(md5($_POST['password']));
//Create query
$qry = "SELECT 'UserName' , 'Password' FROM users WHERE UserName='$UserName' AND Password='$Password'";
$result = mysql_query($qry);
//Check whether the query was successful or not
if($result) {
if(mysql_num_rows($result) > 0) {
//Login Successful
session_regenerate_id();
$member = mysql_fetch_assoc($result);
$_SESSION['SESS_MEMBER_ID'] = $member['mem_id'];
$_SESSION['SESS_FIRST_NAME'] = $member['FName'];
$_SESSION['SESS_LAST_NAME'] = $member['LName'];
//session_write_close();
echo 'SUCCESS';
//loggedin();
//exit();
}
else {
//Login failed
echo 'FAILED.';
//loginfail();
//exit();
}
}
else {
die("Query failed");
}
}
?>
HTML CODE:
<form name="user-form" id="user-form" action="members.php" method="POST">
<input type="text" name="username" id="username" placeholder="Username"></input>
<input type="password" name="password" id="password" placeholder="Password"></input>
<br/>
<input type="submit" id="sign" name="Sign In"></input>
</form>
Your help will be appreciated as I am new to this.

Answer 1

This is wrong-
SELECT 'UserName' , 'Password'....
remove ' and replace with
Also take care of the strings properly, just replace WHERE UserName='$UserName' AND Password='$Password'" with-
WHERE UserName=\"".$UserName."\" AND Password=\"".$Password."\""
So the complete query would be-
"SELECT `UserName` , `Password` FROM users WHERE UserName=\"".$UserName."\" AND Password=\"".$Password."\""
(also keep in mind that the column names are case-sensitive)

Answer 2

This:
$qry = "SELECT 'UserName' , 'Password' FROM users WHERE UserName='$UserName' AND Password='$Password'";
Should be:
$qry = "SELECT username , password FROM users WHERE UserName='$UserName' AND Password='$Password'";