Header already send "webassist/mysqli/queryobj.php on line 687"

Hi.

I keep getting errors when trying to post to my table and I have to encrypt part of the values send and beeing able to decrypt them.

For this purpose I have found the following: https://a1websitepro.com/best-php-encryption-decryption-mysql-tutorial-proactive-methods/

I keep getting this error no matter what I change and how I set the file up.
Link to site: https://amager.skyttesystem.dk/Admin/UserAdm/test2.php

Warning: Cannot modify header information - headers already sent by (output started at /home/skyttesystem/amager.skyttesystem.dk/Admin/UserAdm/test2.php:3) in /home/skyttesystem/amager.skyttesystem.dk/webassist/mysqli/queryobj.php on line 687

For future development (just a suggestion) when setting up mysqli insert / record set it would be nice to have encrypt with (keys you can set elsewhere) to make sure GDPR and such standards could be followed and also easier to use. No complaints, just suggestions and maybe a few video guides on how this can be done. :-)

Hope someone can help with this.

Best Regards
Jess