WP issue, or server issue?

I’m currently unable to test my own plugin (eshop) on my test site: https://eshop.quirm.net/. Here is the snipper of code I’ve narrowed the problem down to:

if(get_option('eshop_checkout')!=''){
 if( $wp_rewrite->using_permalinks()){
  //$p->autoredirect=get_permalink(get_option('eshop_checkout')).'?action=redirect';
  $p->autoredirect=$this_script.'/index.php?page_id='.get_option('eshop_checkout').'&action=redirect';
 }else{
  //$p->autoredirect=get_permalink(get_option('eshop_checkout')).'&action=redirect';
  $p->autoredirect=$this_script.'/index.php?page_id='.get_option('eshop_checkout').'&action=redirect';
 }
}

The $p->autorediect is then added to the action part of a form. eg.
<form method="post" action="https://eshop.quirm.net/index.php?page_id=4&amp;action=redirect">
Until today the plugin has been working fine with the 2 commented out lines, but it appears something has changed somewhere, and I have tried many variations before coming up with the change above. But surely there must be something else wrong.

recently my web hosting went over to php 5 – could this be why the original code no longer works? Or could it be an apache setup difference?

I really am at a loss as to why my original code is no longer working. Especially as it works locally!