PHP site will not work with PHP recommended/dist settings

[HGeneAnthony]

I'm doing some work for a company which uses a PHP inventory program ran by a local shop. Basically, I grabbed a copy of the program from them (because it currently only works on the company's intranet) and I need to add some new features. However, I noticed that the site wouldn't work on my home system properly. What I figured out was that the user was passing variables and not setting them when the page reloaded. For example, he would pass id using a post method and then use it on reload without setting it using:

$id = $_POST['id'];

He would just use it. Now the pages will act like nothing is passed. Unfortunately, he wrote a lot of spaghetti code, which is also uncommented, and I can't find where he passed what. I fixed a few of the variables like the one above but I can't find them all and he also mixed it with JavaScript. I would like to know what I need to set to get his code to work the way it did on the intranat. Any help?