EDIT: SOLVED - Thanks to the comment by @Adam
I have my index.php page, which all included functionality, until now, has worked. I've written a small script (pasted below), using the Simple HTML DOM library to scrape an external webpage for an element value. This value is stored as $newprice inside the script. However, when I place the script anywhere in the code, it a) doesn't run, and b) breaks all HTML/PHP/JS code preceding it.
<?php
include_once('../simple_html_dom.php');
$html = file_get_html('https://www.affordrentacar.co.uk/');
$traverse = $html->find('span.offer-price', 0);
$newprice = $traverse->plaintext;
?>
I've copied this script to one of my other PHP pages and tested echoing out the $newprice variable and that works fine. What could be causing this?