What common error message occurs when attempting to modify header information in PHP?

When attempting to modify header information in PHP, a common error message that occurs is "Warning: Cannot modify header information - headers already sent." This error typically occurs when there is whitespace or output sent before the header() function is called. To solve this issue, make sure that there is no output sent to the browser before calling the header() function. 

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

// Your PHP code here

header("Location: http://www.example.com"); // Modify header information

ob_end_flush(); // Flush output buffer
?>

Keywords

header PHP error message modification output buffers

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