What is the significance of the error message "Cannot modify header information" in PHP?

The error message "Cannot modify header information" in PHP indicates that there was an attempt to modify header information (such as using functions like header() or setcookie()) after some content has already been sent to the browser. To solve this issue, make sure that no output is sent to the browser before calling any functions that modify header information.

ob_start();
// Start output buffering

// Your PHP code here

ob_end_flush();
// Flush the output buffer and send it to the browser