'Call to undefined function' - Trying to call function from an included file - php

Question

So I have two files, 'header.php' and 'pluginfile.php'
The function that I want to call resides in 'pluginfile.php' and is:
public function getNonSubscriptionAmount() {
$total = 0;
foreach($this->_items as $item) {
if(!$item->isSubscription()) {
$total += $item->getProductPrice() * $item->getQuantity();
}
else {
// item is subscription
$basePrice = $item->getBaseProductPrice();
Cart66Common::log('[' . basename(__FILE__) . ' - line ' . __LINE__ . "] Item is a subscription with base price $basePrice");
$total += $basePrice;
}
}
return $total;
}
So in 'header.php' I have:
<?php
include_once($_SERVER['DOCUMENT_ROOT']."/wp-content/plugins/plugin-name/folder/PluginFile.php");
print getNonSubscriptionAmount();
?>
This gives the following error when any page is loaded:
Fatal error: Call to undefined function getnonsubscriptionamount() in
/home/username/domain.com/wp-content/themes/theme/header.php on
line 72
I've spent a couple of hours now trying to figure this out alone and am getting nowhere! Any help much appreciated!

Answer 1

#Wiseguy looks like he had the right idea put in the comments.
You are declaring a method and not a function. Is the function the entirety of your plugin.php file or is there more? If it is everything, remove the public modifier and just declare
function getNonSubscriptionAmount() {
// code here
}
But from the looks of the code it is part of a larger class. If thats the case then #Wiseguy comment is right on, you need to instantiate a new object of the class in plugin.php and then the desired method.
$obj = new PluginClass();
$obj->getNonSubscriptionAmount();

Answer 2

You said:
The function that I want to call resides in 'plugin.php' and is:
And in your file you are including:
So in 'header.php' I have:
include_once($_SERVER['DOCUMENT_ROOT']."/wp-content/plugins/plugin-name/folder/PluginFile.php");
print getNonSubscriptionAmount();
You are not including 'plugin.php' which is were the function lives.
Header.php should include 'plugin.php', not 'PluginFile.php'.