What does the error message "Cannot modify header information - headers already sent" indicate in PHP?

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

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

// Your PHP code here

ob_end_flush(); // Flush output buffer

Keywords

PHP header information headers already sent error message output buffering

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