Query is TRUE when its not - php

Question

Please be gentle with me i have just recently trying to learn PHP/SQL.
The problem is that the first query is ALWAYS TRUE when it shouldn't (base on what i know).
The query simply state to get the 'username' where betakey=$betakey provided by user. The fact that my datebase columns is still empty except column betakey doesn't make that query statement true at all.
Please help, maybe i am missing some knowledge on this.
<?php
header('Access-Control-Allow-Origin: *');
$firstName = $_GET['rfirstname'];
$lastName = $_GET['rlastname'];
$username = $_GET['rusername'];
$password = $_GET['rpass'];
$betakey = $_GET['rkey'];
$host="localhost"; // Host name
$db_username="**"; // Mysql username
$db_password="**"; // Mysql password
$db_name="**"; // Database name
$conn = mysqli_connect("$host", "$db_username", "$db_password","$db_name");
if (!$conn){
die ("Error: ".mysqli_connect_error());
}
$query1 = "SELECT username='$username' FROM users2 WHERE betakey='$betakey';";
$result_1 = mysqli_query($conn,$query1);
if(mysqli_num_rows($result_1) > 0){
echo 'Beta key is used';
}else{
$query2 = "UPDATE users2 SET firstName='$firstName',lastName='$lastName',username='$username',password='$password' WHERE betakey='$betakey'";
echo 'Registration Successful';
}
mysqli_close($conn);//Close off the MySQL connection to save resources.
?>

Answer 1

You have plenty of problems in your code. Let me help you fix some of them
You should learn how to properly open mysqli connection. You need to enable error reporting and set the correct charset.
You should never concatenate PHP variables into SQL query. Always use parameterized prepared statements instead of manually building your queries.
Your first SQL query has an error. username='$username' is meaningless and wrong. If all you want to do is check existence use COUNT(1) or something similar.
Here is my take on your fixed code:
<?php
header('Access-Control-Allow-Origin: *');
$firstName = $_GET['rfirstname'];
$lastName = $_GET['rlastname'];
$username = $_GET['rusername'];
$password = $_GET['rpass'];
$betakey = $_GET['rkey'];
$host = "localhost"; // Host name
$db_username = "**"; // Mysql username
$db_password = "**"; // Mysql password
$db_name = "**"; // Database name
mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$conn = new mysqli($host, $db_username, $db_password, $db_name);
$conn->set_charset('utf8mb4');
$stmt = $conn->prepare("SELECT COUNT(username) FROM users2 WHERE betakey=?");
$stmt->bind_param('s', $_GET['rusername']);
$stmt->execute();
$result_1 = $stmt->get_result();
$used = $result_1->fetch_row()[0];
if ($used) {
echo 'Beta key is used';
} else {
$stmt = $conn->prepare("UPDATE users2 SET firstName=?, lastName=?, username=?, password=? WHERE betakey=?");
$stmt->bind_param('sssss', $firstName, $lastName, $username, $password, $betakey);
$stmt->execute();
echo 'Registration Successful';
}