Inserting css source code inside the php file itself

I am trying to insert a CSS code inside the php file itself. But I am encountering with an error continuously when even I update my post. Error is:

Cannot modify header information - headers already sent by (output started at bla bla on line 32) in /home/bla bla/wp-admin/post.php on line 235

Below is the code I am adding inside my php file:

echo "";
echo    "";