wp admin Warning: Cannot modify header information – headers already sent by

have you checked .htaccess file too? not sure if a space/tab there can cause this issue too.

@lailiz , thankyou very much for your reply

but this problem has been solved by replacing wp-config.php with the fresh one and reconfigure it .

this is what i’m doing :
1.rename your wp-config.php to wp-config-backup.php
2.open wp-config-sample.php
3.editing it as required, and then saving it as wp-config.php

i dont know exactly what the problem inside my wp-config.php that make warning message appear but this method help me out from this problem.

I Just had the same problem

Note: I previously modified my themes functions.php file by coping example code directly into the file.

The example code included the php open tag <?php and close tag ?>
everything worked ok until I had to do a restart.

I solved the problem by removing all php closing tags and all but the first opening php tag.

All the errors went away.