Basic Template Engine Fix - php

Question

I am attempting to make a (very) basic template engine for php. Based on my research I have found that a method that I am using is strongly disliked. I was wondering if anyone knew a great alternative to get the same result so I am not using it. And if anyone sees any other improvements that can be made please share!
the method that is not advised is the eval() method!
Here is the php file
<?php
class Engine {
private $vars = array();
public function assign($key, $value) {
$this->vars[$key] = $value;
}
public function render($file_name) {
$path = $file_name . '.html';
if (file_exists($path)) {
$content = file_get_contents($path);
foreach ($this->vars as $key => $value) {
$content = preg_replace('/\{' . $key . '\}/', $value, $content);
}
eval(' ?>' . $content . '<?php ');
} else {
exit('<h4>Engine Error</h4>');
}
}
}
?>
here is the index.php file
<?php
include_once 'engine.php';
$engine = new Engine;
$engine->assign('username', 'Zach');
$engine->assign('age', 21);
$engine->render('test');
?>
and here is just a test html file to display its basic function
My name is {username} and I am {age} years old!
outputs:
My name is Zach and I am 21 years old!
Many thanks in advance!

Answer 1

If you just want to output some text to the page, just use echo:
echo $content;
This is better than eval('?>' . $content . '<?php') for quite a few reasons: for one, if someone types in <?php phpinfo(); ?>, for example, as their username, it won't execute that code.
I would, however, note that you have some other problems. What if I do this?
$engine = new Engine;
$engine->assign('username', '{age}');
$engine->assign('age', 21);
$engine->render('test');
The {age} in the username value will be replaced with 21. Usually you don't want replacements to be replaced like that, particularly as it's order-dependent (if you assigned username later, it wouldn't happen).