What potential issues can arise from mixing PHP code and plain text in the same file?

Mixing PHP code and plain text in the same file can lead to readability issues, maintenance difficulties, and potential security vulnerabilities. To solve this problem, it's best to separate the PHP code from the plain text by using a template engine like Smarty or Twig. This approach allows for a cleaner separation of logic and presentation, making the code easier to maintain and understand.

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

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

$variable = 'Hello, World!';

echo $twig->render('index.html', ['variable' => $variable]);
```

In this example, we're using the Twig template engine to render the 'index.html' template file, which contains the plain text content. The PHP code is kept separate from the plain text, making the code easier to read and maintain.