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What are the best practices for handling form submissions in PHP to prevent data manipulation or unauthorized access?

To prevent data manipulation or unauthorized access when handling form submissions in PHP, it is crucial to validate and sanitize user input before processing it. This includes checking for required fields, validating input formats, and using prepared statements to prevent SQL injection attacks. Additionally, implementing CSRF tokens can help protect against cross-site request forgery attacks.

php 
```php
// Validate and sanitize user input
$name = isset($_POST['name']) ? htmlspecialchars($_POST['name']) : '';
$email = isset($_POST['email']) ? filter_var($_POST['email'], FILTER_SANITIZE_EMAIL) : '';
// Check for required fields
if(empty($name) || empty($email)) {
    // Handle error
}
// Validate email format
if(!filter_var($email, FILTER_VALIDATE_EMAIL)) {
    // Handle error
}
// Use prepared statements to prevent SQL injection
$stmt = $pdo->prepare("INSERT INTO users (name, email) VALUES (:name, :email)");
$stmt->bindParam(':name', $name);
$stmt->bindParam(':email', $email);
$stmt->execute();
// Implement CSRF tokens
session_start();
$token = bin2hex(random_bytes(32));
$_SESSION['csrf_token'] = $token;
```
Note: This code snippet demonstrates how to validate and sanitize user input, check for required fields, use prepared statements to prevent SQL injection, and implement CSRF tokens in PHP form submissions.

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