What are the differences in PHP versions (4/5) that could affect the functionality of a custom template engine?

The main difference between PHP 4 and PHP 5 that could affect the functionality of a custom template engine is the introduction of object-oriented programming features in PHP 5. If the custom template engine relies heavily on object-oriented programming principles, it may not work properly in PHP 4 due to the lack of support for classes and objects. To solve this issue, the custom template engine code needs to be updated to be compatible with PHP 4 by removing any object-oriented features and using procedural programming instead.

// PHP 4 compatible code snippet
// Replace any object-oriented code with procedural code

// PHP 4 compatible class definition
class TemplateEngine {
    var $template;

    function TemplateEngine($template) {
        $this->template = $template;
    }

    function render($data) {
        // Render template using procedural code
    }
}

// PHP 4 compatible usage of the template engine
$template = new TemplateEngine('template.php');
$template->render($data);