What are the consequences of having output before session_start() or setcookie() functions in PHP scripts?

Having output before session_start() or setcookie() functions in PHP scripts can lead to headers already being sent error messages. To solve this issue, make sure that there is no output (including whitespace) before calling session_start() or setcookie() functions in your PHP scripts. 

<?php
ob_start(); // Start output buffering
session_start(); // Start the session
// Your PHP code here
ob_end_flush(); // Flush the output buffer
?>

Keywords

output buffering headers already sent session data loss cookie not set PHP script error

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