What does the error "Warning: Cannot modify header information - headers already sent by" in PHP mean?

The error "Warning: Cannot modify header information - headers already sent by" in PHP means that there was some output (such as HTML, whitespace, or error messages) sent to the browser before the PHP header() function was called. To solve this issue, make sure that there is no output sent before calling the header() function, and that the header() function is called before any output is sent to the browser. 

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

// Your PHP code here

header('Location: new_page.php'); // Redirect to a new page

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

Keywords

PHP header output buffering ob_start error

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