setcookie, Cannot modify header information - headers already sent

The error message "Cannot modify header information - headers already sent" typically occurs when a PHP script attempts to send a cookie or redirect the user's browser to a different page, but some output has already been sent to the browser. This could be caused by any of the following:

• whitespace or other characters before the opening PHP tag in the script
• an echo, print, or other statement that sends output to the browser before the setcookie() or header() function is called
• a PHP extension or other software that automatically adds headers to the response, such as an output buffer handler

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Example:

<?php

// This will cause the error because output has already been sent to the browser
echo "Hello, world!";

// This will not work because headers have already been sent
setcookie("test", "value");

To fix the issue, you should ensure that no output is sent to the browser before calling setcookie() or header(). This can be done by removing any whitespace or other characters before the opening PHP tag, moving echo or other statements that send output to the browser after setcookie() or header() is called, or disabling any PHP extensions or other software that automatically adds headers to the response.