How can the issue of "Headers already sent" be resolved after using require_once in PHP?

The issue of "Headers already sent" in PHP occurs when there is output (such as whitespace) before header functions like header() or setcookie() are called. To resolve this issue after using require_once, you can make sure there is no output before header functions by using ob_start() to buffer the output. 

<?php
ob_start();
require_once 'file.php';
ob_end_clean();

Keywords

require_once Headers already sent output buffering ob_start ob_end_clean

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