Happens when your script tries to send a HTTP header to the client but there already was output before, which resulted in headers to be already sent to the client.
Possible Causes :
Print, echo: Output from print and echo statements will terminate the opportunity to send HTTP headers. The application flow must be restructured to avoid that.
Raw HTML areas: Unparsed HTML sections in a .php file are direct output as well. Script conditions that will trigger a
header() call must be noted before any raw blocks.
<!DOCTYPE html> <?php // Too late for headers already.
<?php for "script.php line 1" warnings: If the warning refers to output in line 1, then it's mostly leading whitespace, text or HTML before the opening
<?php # There's a SINGLE space/newline before <? - Which already seals it.