login.php

$username = $_POST['username'];
$password = $_POST['password'];

$query_teacher = mysqli_query($db, "SELECT * FROM teacher WHERE username='$username' AND password='$password'")or die(mysqli_error($db));
$row_teacher = mysqli_fetch_assoc($query_teacher);
$num_row_teacher = mysqli_num_rows($query_teacher);

if ($num_row_teacher > 0){
    if (password_verify($password, $row_teacher['password'])) {
        echo 'true';
    } else {
        echo 'false';
    }
} else {
    echo 'false';
}