Sure! Here are 9 points on how PHP can be used to create secure web applications:

1. **Input validation**: Proper input validation is critical to prevent attacks such as SQL injection, cross-site scripting (XSS), and cross-site request forgery (CSRF). For example, PHP's filter_var() function can be used to validate and sanitize user input, such as email addresses, URLs, and integers.

2. **Prepared statements**: Prepared statements can help prevent SQL injection attacks by separating SQL logic from user input. PHP's PDO (PHP Data Objects) and mysqli extensions provide support for prepared statements. For example:
```php
$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :username');
$stmt->bindParam(':username', $username);
$stmt->execute();
```

3. **Password hashing**: Storing passwords securely is crucial. PHP provides functions like password_hash() and password_verify() to securely hash and verify passwords using industry-standard algorithms like bcrypt. For example:
```php
// Hash password
$password = 'mypassword';
$hashedPassword = password_hash($password, PASSWORD_DEFAULT);

// Verify password
if (password_verify($password, $hashedPassword)) {
    // Password is correct
} else {
    // Password is incorrect
}
```

4. **Session management**: Proper session management is essential to prevent session hijacking and session fixation attacks. PHP provides built-in session management functions like session_start() and session_regenerate_id() to manage user sessions securely. For example:
```php
// Start a session
session_start();

// Set session variables
$_SESSION['user_id'] = 12345;

// Regenerate session ID
session_regenerate_id(true);
```

5. **Cross-Site Scripting (XSS) prevention**: PHP provides functions like htmlspecialchars() and htmlentities() that can be used to properly escape user input to prevent XSS attacks. For example:
```php
// Display user input safely
echo htmlspecialchars($userInput, ENT_QUOTES, 'UTF-8');
```

6. **Cross-Site Request Forgery (CSRF) protection**: PHP can be used to implement CSRF protection tokens to prevent unauthorized requests from being executed on behalf of a user. For example:
```php
// Generate CSRF token
$csrfToken = bin2hex(random_bytes(32));

// Store CSRF token in session
$_SESSION['csrf_token'] = $csrfToken;

// Include CSRF token in form
<input type="hidden" name="csrf_token" value="<?php echo $csrfToken; ?>">
```

7. **File upload validation**: If your web application allows file uploads, it's crucial to validate and sanitize file uploads to prevent file-based attacks like file inclusion vulnerabilities and file upload exploits. For example, PHP's fileinfo extension can be used to validate file types and ensure that uploaded files are safe.

8. **Secure database connections**: PHP allows for establishing secure database connections using SSL or TLS to encrypt data transmitted between the web application and the database server. For example, using the "mysqli" extension:

```php
// Connect to MySQL database with SSL
$mysqli = new mysqli("localhost", "username", "password", "database", null, '/path/to/ca-cert.pem');
```

9. **Regular updates and patching**: Keeping PHP and all its libraries up-to-date with the latest security patches is crucial to protect against known vulnerabilities. Regularly checking for updates and applying them promptly can help ensure the security of your PHP web application.

Remember, PHP is just one part of building a secure web application. Other best practices such as server hardening, using HTTPS, implementing proper access controls, and user authentication also play a critical role in creating a secure web application. It's important to follow security best practices at every level of your web application.
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