Implement Comprehensive Server-Side Input Validation

function validateUserData($data) {
    $errors = [];

    // Validate 'username'
    if (empty($data['username'])) {
        $errors[] = "Username is required.";
    } elseif (!preg_match("/^[a-zA-Z0-9_]{3,20}$/", $data['username'])) {
        $errors[] = "Username must be 3-20 characters long and contain only letters, numbers, or underscores.";
    }

    // Validate 'email'
    if (empty($data['email'])) {
        $errors[] = "Email is required.";
    } elseif (!filter_var($data['email'], FILTER_VALIDATE_EMAIL)) {
        $errors[] = "Invalid email format.";
    }

    // Validate 'age'
    if (empty($data['age'])) {
        $errors[] = "Age is required.";
    } elseif (!filter_var($data['age'], FILTER_VALIDATE_INT, ["options" => ["min_range" => 18, "max_range" => 120]])) {
        $errors[] = "Age must be an integer between 18 and 120.";
    }

    // Validate 'password' (basic length check, not hashing)
    if (empty($data['password'])) {
        $errors[] = "Password is required.";
    } elseif (strlen($data['password']) < 8) {
        $errors[] = "Password must be at least 8 characters long.";
    }

    return $errors;
}

// Example usage:
$post_data = [
    'username' => 'test_user123',
    'email' => '[email protected]',
    'age' => 25,
    'password' => 'securePass123'
];

$validation_errors = validateUserData($post_data);

if (!empty($validation_errors)) {
    echo "Validation Errors:
" . implode("
", $validation_errors);
} else {
    echo "Data is valid. Proceed with processing.
";
    // Further processing (e.g., save to database)
}

$invalid_data = [
    'username' => 'ab',
    'email' => 'invalid-email',
    'age' => 15,
    'password' => 'short'
];
$invalid_errors = validateUserData($invalid_data);
if (!empty($invalid_errors)) {
    echo "
Validation Errors for invalid data:
" . implode("
", $invalid_errors);
}