PHP plugin system

So every PHP plugin system I've seen is OO.
Is there any way to make a plugin system using functions?

>>2
A system that for example, has places in the code where plugins can put their own code in.

index.php
<?php
hi();
?>

plugin.php
<?php
echo "hi";
?>

The hi() in index.php gets replaced with echo "hi" in plugin.php.