Appearance :
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 :
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
token.<?php
# There's a SINGLE space/newline before <? - Which already seals it.