mysqli conversion PDO - php

Question

How can I turn this to PDO? I tried, but I don't know how.
It was MySQLi at first and I tried to turn it to PDO and so, that was the result:
<?php
$username = $_POST['username'];
$password = $_POST['password'];
$stmt = $conn-> prepare("SELECT * FROM resident WHERE '$username' = ? AND '$password' = ?");
//mysqli_stmt_bind_param($sql, "ss", $username, $password);
$stmt->bindParam(':username', $username);
$stmt->bindParam(':password', $password);
$stmt ->execute();
mysqli_stmt_store_result($sql);
$stmt-> BindParam($sql, $name, $username, $password);
$response=array();
$response["succes"] = false;
while (mysqli_stmt_fetch($sql)) {
$response["success"] = true;
$response["name"]= $name;
}
echo json_encode ($response);
?>

Answer 1

You're confusing variables with column names. It's extremely important to note the difference. In a query '$username' is a string with a value in it, and probably a SQL injection bug. username without quotes is probably a column name.
You're also using named placeholders but you haven't named them. ? is an unnamed one. If you want a placeholder named :x then :x must appear in the query.
Your fixed code should look like:
$stmt = $conn-> prepare("SELECT * FROM resident WHERE username=:username AND password=:password");
//mysqli_stmt_bind_param($sql, "ss", $username, $password);
$stmt->bindParam(':username', $username);
$stmt->bindParam(':password', $password);
$stmt ->execute();
That is technically functional but VERY, VERY WRONG. Passwords must be properly hashed using, at the absolute least, password_hash. If you do that you can no longer fetch based on username and password, you need to fetch based on username and use password_verify to check if it's correct or not.
Disclaimer: Don't Write Your Own Login System
Unless this is strictly for academic purposes, all of this code is pretty much a waste of time. Any development framework has a solution for this. One example is Laravel where out of the box you get a full-featured authentication system.

Answer 2

You can use answer of tadman, also this:
$stmt = $conn-> prepare("SELECT `id` FROM resident WHERE username=:username AND password=:password");
$stmt->execute(array(':username' => $username, ':password' => $password);
And hash your passwords.
You can learn some basics of pdo Here, W3schools