What is the main issue with the PHP script described in the forum thread?

The main issue with the PHP script described in the forum thread is that it is vulnerable to SQL injection attacks due to directly concatenating user input into the SQL query. To solve this issue, you should use prepared statements with parameterized queries to safely handle user input. 

// Original vulnerable code
$username = $_POST['username'];
$password = $_POST['password'];

// Vulnerable SQL query
$sql = "SELECT * FROM users WHERE username = '$username' AND password = '$password'";

// Fixed code using prepared statements
$stmt = $pdo->prepare("SELECT * FROM users WHERE username = :username AND password = :password");
$stmt->execute(['username' => $username, 'password' => $password]);

Keywords

syntax error undefined variable function call database connection debugging techniques

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