How can PHP developers ensure proper separation between PHP and HTML code when generating dynamic content?

To ensure proper separation between PHP and HTML code when generating dynamic content, PHP developers can use a templating engine like Twig or Blade. These templating engines allow developers to write clean and maintainable code by separating the presentation logic (HTML) from the business logic (PHP).

```php
// Example using Twig templating engine
require_once 'vendor/autoload.php';

$loader = new \Twig\Loader\FilesystemLoader('templates');
$twig = new \Twig\Environment($loader);

$dynamicData = [
    'name' => 'John Doe',
    'age' => 30
];

echo $twig->render('index.html', $dynamicData);
```

In this example, the Twig templating engine is used to render the 'index.html' template with the dynamic data provided in the $dynamicData array. This allows for a clean separation between PHP and HTML code.