What is the significance of the error message "Cannot send session cookie - headers already sent" in PHP?

The error message "Cannot send session cookie - headers already sent" in PHP indicates that there was output sent to the browser before session_start() was called, which prevents PHP from setting session cookies. To solve this issue, make sure that session_start() is called before any output is sent to the browser, including any whitespace characters. 

<?php
// Start the session before any output is sent
session_start();

// Rest of your PHP code goes here
?>

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session cookie headers PHP error message significance

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