You are moving the code from one enviroment to another.
Many things can go wrong. "The code is not working" is vague.
You can create a file, call it, for example, "phpinfo.php", and as contents, write this inside it:
<?php phpinfo() ?>
And run it on both environments. It will give you the version, and configuation variables, and you can compare.
Some of the things that can go wrong:
- Different versions of PHP: sometimes it's a problem, sometimes it's not. It depends on the used features
- the database not configured correctly
- some libraries (for example GD2) not being enabled
- using absolute paths and URLs in the code (bad practice)
- using "PHP safe mode", some stupid hosts still do that
- file permission issues in some directories
- short open tags: depeding on the configuation, "<?" will be accepted as an alternative to "<?php" or not...
and other things!
As you may start to guess by now, your best bet is to get in touch with the developer.