Checking opt:content with more complex expressions.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:root>
<p opt:content="$c+$d">Default content</p>
<p opt:content="$a+$b">Default content</p>
</opt:root>

>>>>expected.txt
OUTPUT

>>>>data.php
$view->a = 5;
$view->b = 3;

>>>>result.txt
<p>Default content</p>
<p>8</p>