How can I include functions that return echo values to my styles?

[ThorstenA]

There are functions that return echo values, how can I integrate them with style where I have a $content variable?

function sayhello()
{
	echo 'hello';

}

I tried this but it did not work.

eval('$content .= "' .  sayhello() .  '";');

$content .=  sayhello();