What version of Zen Cart are you using?
Was this an upgrade from a prior version? Which version?
What addons/contributions are installed on your site?
What version of PHP is running on the server?
Who is your hosting company?

When did these errors start happening? How does the "when" correlate to anything else related to the website's operations?

Is there anything that's not functioning properly due to these errors?
You said the store is functional. Are you therefore saying that these errors are not preventing customers from shopping?

One which screens do you see this error displayed?
Under what conditions does it occur?
What exact steps can be taken to reproduce this error on your site?
What is the URL where it can be seen in action?

If you add this to the top of your /index.php file (in your main Zen Cart folder) and attempt to create the error again, what does it show on the top of the page before the Variable passed to each() error appears ?:
Code:
echo '<pre>' . print_r($_POST, true) . '</pre>';
(You're going to see some "headers already sent" warnings, which can be ignored while doing this testing)