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

The error message "Cannot add header information - headers already sent" in PHP occurs when you try to set a header using the header() function after output has already been sent to the browser. To solve this issue, make sure to set headers before any output is sent to the browser, such as HTML content or whitespace. 

<?php
ob_start(); // start output buffering
// set headers here
header("Content-Type: text/html");

// your PHP code here

ob_end_flush(); // flush output buffer
?>

Keywords

PHP header output buffering ob_start error message

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