What is the common error message "Cannot modify header information - headers already sent by" in PHP and how can it be resolved?

The common error message "Cannot modify header information - headers already sent by" in PHP occurs when there is output (such as echo, print, or whitespace) before the header() function is called. To resolve this issue, make sure that there is no output sent to the browser before calling header(). 

<?php
ob_start(); // Start output buffering

// Your PHP code here

ob_end_flush(); // Flush output buffer and send content to the browser

Keywords

PHP header output buffering ob_start ob_end_clean

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