What is the significance of the error message "Cannot modify header information - headers already sent" in PHP?

The error message "Cannot modify header information - headers already sent" in PHP occurs when there is output sent to the browser before PHP attempts to modify header information. To solve this issue, make sure there is no whitespace or output (such as echo, print, or HTML) before any header() function calls in your PHP script. 

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

// Your PHP code here

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

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PHP header information headers already sent error message significance

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