我需要帮助来解决这个问题。我想登录，但我遇到了这个错误。
<?php
require_once 'includes/config.php';
$username = $password = "";
$username_err = $password_err = "";
if($_SERVER["REQUEST_METHOD"] == "POST")
{
if(empty(trim($_POST["username"])))
{
$username_err = 'Please enter username.';
}
else
{
$username = trim($_POST["username"]);
}
if(empty(trim($_POST['password'])))
{
$password_err = 'Please enter your password.';
}
else
{
$password = trim($_POST['password']);
}
if(empty($username_err) && empty($password_err))
{
$sql = "SELECT username, password FROM students WHERE username = ?";
if($stmt = mysqli_prepare($link, $sql))
{
mysqli_stmt_bind_param($stmt, "s", $param_username);
$param_username = $username;
if(mysqli_stmt_execute($stmt))
{
mysqli_stmt_store_result($stmt);
if(mysqli_stmt_num_rows($stmt) == 1)
{
mysqli_stmt_bind_result($stmt, $username, $hashed_password);
if(mysqli_stmt_fetch($stmt))
{
if(password_verify($password, $hashed_password))
{
session_start();
$_SESSION['username'] = $username;
header("location: welcome.php");
}
else
{
$password_err = 'The password you entered was not valid.';
}
}
}
else
{
$username_err = 'No account found with that username.';
}
}
else
{
echo "Oops! Something went wrong. Please try again later.";
}
}
mysqli_stmt_close($stmt);
}
mysqli_close($link);
}
?>