What is the significance of the error message "Cannot modify header" in PHP and how does it affect the functionality of a website?

The error message "Cannot modify header information - headers already sent" in PHP occurs when there is output sent to the browser before header functions are called. This can happen due to whitespace, HTML tags, or any other output before calling functions like header(). To solve this issue, ensure that there is no output sent before calling header functions. 

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

// Your PHP code here

ob_end_flush(); // Flush output buffer

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header PHP error message functionality website

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