Warning: Cannot modify header information – headers already sent by (output star

i face this error every time i am trying to add new page or post
Warning: Cannot modify header information – headers already sent by (output started at /home/integr76/public_html/study/wp-content/themes/idg/functions.php:58) in /home/integr76/public_html/study/wp-includes/pluggable.php on line 866
any help please
thanks