First of all it is not recommended to have php files with functions in design folder. You should create a new module or extend (copy from core to local a helper and add function onto that class) and do not change files from app/code/core.
To answer to your question you can use:
require(Mage::getBaseDir('design').'/frontend/default/mytheme/myfunc.php');
Best practice (as a start) will be to create in /app/code/local/Mage/Core/Helper/Extra.php a php file:
<?php
class Mage_Core_Helper_Extra extends Mage_Core_Helper_Abstract
{
public function getSomething()
{
return 'Someting';
}
}
And to use it in phtml files use:
$this->helper('core/extra')->getSomething();
Or in all the places:
Mage::helper('core/extra')->getSomething();