What are common reasons for the "headers already sent" error when using session_start() in PHP?

The "headers already sent" error in PHP occurs when there is any output (such as whitespace or HTML) sent to the browser before the session_start() function is called. To solve this issue, make sure there is no output before session_start() is called, and ensure that session_start() is the first thing called in your PHP script. 

<?php
ob_start(); // Start output buffering
session_start(); // Start the session

// Rest of your PHP code goes here

ob_end_flush(); // Flush the output buffer
?>

Keywords

session_start headers already sent PHP error reasons

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