How To fixed this Errors "Warning: Cannot modify header information" and Undefined Problems [duplicate]

This question already has an answer here:

• What does the PHP error message “Notice: Use of undefined constant” mean? 9 answers
• How to fix “Headers already sent” error in PHP 11 answers

i am working on my own project . but i am stuck on abig problems. Please Help me . My Problems are :

Warning: Use of undefined constant eid - assumed 'eid' (this will throw an Error in a future version of PHP) in /opt/lampp/htdocs/new2c/account.php on line 455

Warning: Use of undefined constant n - assumed 'n' (this will throw an Error in a future version of PHP) in /opt/lampp/htdocs/new2c/account.php on line 455

Warning: Use of undefined constant t - assumed 't' (this will throw an Error in a future version of PHP) in /opt/lampp/htdocs/new2c/account.php on line 455

Warning: Cannot modify header information - headers already sent by (output started at /opt/lampp/htdocs/new2c/account.php:22) in /opt/lampp/htdocs/new2c/account.php on line 455

Notice: Undefined index: total in /opt/lampp/htdocs/new2c/account.php on line 490

Notice: Undefined index: total in /opt/lampp/htdocs/new2c/account.php on line 511

and 2nd problems are :

Warning: Cannot modify header information - headers already sent by (output started at /opt/lampp/htdocs/new2c/account.php:22) in /opt/lampp/htdocs/new2c/account.php on line 403

My codes:

account.php: https://ideone.com/Num0qp
qupdate.php: https://ideone.com/bFL2y5

Thanks in Advanced.