Get the Results of an Include in a String in PHP?

Let's say file test.php looks like this:

<?php
echo 'Hello world.';
?>

I want to do something like this:

$test = include('test.php');

echo $test;

// Hello world.

Can anyone point me down the right path?

Edit:

My original goal was to pull PHP code intermingled with HTML out of a database and process it. Here's what I ended up doing:

// Go through all of the code, execute it, and incorporate the results into the content
while(preg_match('/<\?php(.*?)\?>/ims', $content->content, $phpCodeMatches) != 0) {
    // Start an output buffer and capture the results of the PHP code
    ob_start();
    eval($phpCodeMatches[1]);
    $output = ob_get_clean();

    // Incorporate the results into the content
    $content->content = str_replace($phpCodeMatches[0], $output, $content->content);
}