PHP - How Would I Check If User Is Pending Already?

<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick-subscriptions">
<input type="hidden" name="business" value="mysandboxemail@gmail.com">
<input type="hidden" name="currency_code" value="GBP">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="cbt" value="Return to The Store">
<input type="hidden" name="cancel_return" value=" <?php echo $actual_link ?>">
<input type="hidden" name="custom" value="This is a custom field!!!">
<input type="hidden" name="item_name" value="ACCOUNT UPGRADE">

<input type="hidden" name="a3" value="5.00">
<input type="hidden" name="p3" value="1"> 
<input type="hidden" name="t3" value="M">
<input type="hidden" name="src" value="1">
<input type="hidden" name="sra" value="1">
<input type="image" src="http://www.paypal.com/en_US/i/btn/btn_subscribe_LG.gif" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
</form>

$usrsql = "SELECT * FROM $tbl_name WHERE username='$username' AND password='$password'";
        $usrres = mysql_query($usrsql);
        $usrcount = mysql_num_rows($usrres);

        if($usrres && mysql_num_rows($usrcount)>0) {

            die("Username is already taken!");
        } 

function Login($username, $password)
	{
		try
		{
			$db = new PDO("mysql:host=".DB_HOST.";dbname=".DB_NAME.";charset=utf8", DB_USERNAME, DB_PASSWORD);
			$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
			$db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
		}
		catch(PDOException $ex)
		{
			echo "Unable to connect to DB";
			error_log($ex->getMessage());
		}
		
		try
		{
			$User_Info = $db->prepare("SELECT * FROM users WHERE username=:username");
			$User_Info->bindValue(":username", $username, PDO::PARAM_STR);
			$User_Info->execute();
			$Info = $User_Info->fetchAll(PDO::FETCH_ASSOC);
			
			$salt = $Info['salt'];
			
			$password = $salt . $password;
			$password = $this->CreateHash($password);
			
			$unique_key = $this->GenerateRandom();
			$unique_key = $this->CreateHash($unique_key);		
			
			$Check_User = $db->prepare("SELECT * FROM users WHERE username=:username AND password=:password");
			$Check_User->bindValue(":username", $username, PDO::PARAM_STR);
			$Check_User->bindValue(":password", $password, PDO::PARAM_STR);
			$Check_User->execute();	
			
			if($Check_User->rowCount() > 0)
			{
				while($row = $Check_User->fetchAll(PDO::FETCH_ASSOC))
				{
					session_destroy();
					session_start();
					
					$_SESSION = array();
					
					session_regenerate_id(true);
					
					$_SESSION['username'] = $row['username'];
					$session_id = session_id();
					$user_id = $row['id'];
					
					$Check_Logged_In = $db->prepare("DELETE FROM logged_in_users WHERE user_id=:userid");
					$Check_Logged_In->bindValue(":user_id", $user_id, PDO::PARAM_STR);
					$Check_Logged_In->execute();
					$has_changed = $Check_Logged_In->rowCount();
					
					if($has_changed > 0)
					{
						$Logged_In = $db->prepare("INSERT INTO logged_in_users (id, user_id, session_id, unique_key) VALUES (NULL, :user_id, :session_id, :unique_key)");
						$Logged_In->bindValue(":user_id", $user_id, PDO::PARAM_STR);
						$Logged_In->bindValue(":session_id", $session_id, PDO::PARAM_STR);
						$Logged_In->bindValue(":unique_key", $unique_key, PDO::PARAM_STR);
						$Logged_In->execute();
						$affected_rows = $Logged_In->rowCount();
						
						if($affected_rows > 0)
						{
							return true;
						}
					}
					return false;
				}
			}

			return false;
		}
		catch(PDOException $ex)
		{
			echo "Unable to complete query";
			error_log($ex->getMessage());
		}
	}