What does the warning "Cannot modify header information - headers already sent" mean in PHP?

The warning "Cannot modify header information - headers already sent" in PHP occurs when your script tries to send HTTP headers after it has already sent output to the browser. This can happen if there is whitespace or text output before the header function calls. To solve this issue, make sure that there is no output sent to the browser before calling functions like header() or setcookie(). 

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

// Your PHP code here

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

Keywords

PHP header warning output buffering ob_start

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