Having now used two different hosting companies for PHP web site hosting, I would like to quick describe the configuration issue that have occured with both hosting companies. They both set PHP's register_globals configuration variable to On, which can have undesirable effects on your application and make you easily second guess why your application is behaving in a certain way.
When register_globals is set to On, this means that your variables are registered globally and available on every page. So if you declare a variable called $userCount and assign it some value in a page called script1.php and then have another variable with the same name ($userCount) but with a different meaning on another page (say script2.php), it will initially take the value assigned when script1.php was executed!
So let's say script1.php performed the following assignment:
$userCount = 5;
When script2.php executes, if it also has a variable called $userCount, its initial value will be 5! Very confusing!
So take my advice, if you can configure PHP yourself on your web hosting site, then set register_globals to Off, or ask your web hosting company to do this for you.
Also note that the register_globals configuration variable has been removed as of PHP version 5.4.