What are some common vulnerabilities, such as XSS and SQL injection, that developers should be aware of when implementing user authentication in PHP?

1. Cross-Site Scripting (XSS): XSS occurs when an attacker injects malicious scripts into web pages viewed by other users. To prevent XSS attacks, developers should sanitize user input and encode output to prevent script execution. 

$user_input = $_POST['username'];
$sanitized_input = htmlspecialchars($user_input, ENT_QUOTES, 'UTF-8');
echo $sanitized_input;
```

2. SQL Injection: SQL injection is a common attack where malicious SQL queries are inserted into input fields to manipulate databases. To prevent SQL injection, developers should use prepared statements or parameterized queries to sanitize user input before executing SQL queries.

```php
$user_input = $_POST['username'];
$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :username');
$stmt->bindParam(':username', $user_input);
$stmt->execute();

Keywords

XSS SQL injection user authentication PHP vulnerabilities

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