Please help us help you better by showing us the following elements:

Show us the part of your code where you delete the cookie.

Please also show us the output including the headers which your script gives when deleting the cookie.

Please also show us what your research has shown that is necessary to delete a cookie.

Please also show us the steps that are necessary to change your script from writing the current headers to writing the headers that your research has shown you need.

Please tell us where in the process you encounter problems.