Warning: Cannot modify header information - headers already sent
Happens when your script tries to send an HTTP header to the client but there already was output before, which resulted in headers to be already sent to the client.
This is an E_WARNING and it will not stop the script.
A typical example would be a template file like this:
<html>
...
02 September, 2020
How to do an INNER JOIN on multiple columns
You can JOIN with the same table more than once by giving the joined tables an alias, as in the following example:SELECT airline, flt_no, fairport, tairport, depart, arrive, fareFROM flightsINNER JOIN airports from_port ON (from_port.code = flights.fairport)INNER JOIN airports to_port ON (to_port.code = flights.tairport)WHERE ...