Fix Header Already Send Issue In Php

im have been getting this error for a while now on my php script

how do i fix header already send issue in php?

session_start(): Cannot send session cache limiter - headers already sent


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How To Fix Error Warning: session_start(): Cannot send session cache limiter - headers already sent
By: Help Fix Code Script

I was migrating a website from one server to another server. Once I had uploaded all the files to the new server and pulled the website, I kept getting this error at the top of the page:

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /var/www/html/includes/myfile.php:1) in /var/www/header_file.php on line 7

when I pulled header_file.php, I had this on line 7:

session_start();