Shopping Cart Empty or PHP Session Not Working Correctly

I have built a WordPress site with a shopping cart. Whenever a buyer attempts to make a purchase, we are getting an error
“Either your shopping cart is empty or the PHP Session on your server is not working correctly.”

I have checked with our hosting who verified the PHP sessions are working correctly. They advised that running a simple session start command in a test script(which I do not know how to do) will show this to me. Or, perhaps the wrong type of sessions are selected in the cart. I have no idea how to fix this and am losing customers because of my limited knowledge
of PHP sessions. Can anyone suggest some corrective measures or where I can look for help? Thanks so much!

The session start command that your hosting is refering to is:


// ... the rest of the script


You can’t have any output being sent before the use of session_start()