“Cannot modify header information – headers already sent by” error is most common PHP error happen when we develop a web application. Usually when we want to redirect or send cookie. I used to get that warning a lot long time ago. But since i standardize my PHP code, it doesn’t happen for a while now, until last night.
Yesterday night i was fixing somebody else code and audit the performance as well as the security. But when testing it, i got very strange error, the error was “Headers already sent by” at line 1. I open the file and see at the line 1, it was nothing there, it is just a php open tags, and nothing else.
It confuse me for 1 hour until i finally got the answer from this stackoverflow. The problem is there is a white space unseen character before the oepn php tags. To check it i’m using Notepad++ with plugin called Hex Editor. And i found out there is invisible character at the beginning of the file.
I clean it, check every file and voila, everything works like a charm. So if you found any strange errors, and come to this article, it worth to check it out.