Warning: Cannot modify header information - headers already sent by (output started at

Here is the reason why this error occured.

1. Please check Whitespace before <?php or after ?>

2. No output before sending headers. Functions that send/modify HTTP headers must be invoked before any output is made. 

3. Some functions modifying the HTTP header are:

header / header_remove

session_start / session_regenerate_id

setcookie / setrawcookie

4. The UTF-8 Byte Order Mark specifically

5. Previous error messages or notices

6. print, echo and other functions producing output