Warning: Unable to modify header information - header already started sending output at line 12 of /some/file.php at line 23 of /some/file.php

Question

To understand why headers must be sent before output, you need to look at a typical HTTP response. The PHP script mainly generates HTML content and also passes a set of HTTP/CGI headers to the web server. This error occurs when you host a website with a database or PHP site on the server. First, you need to go into your hosting website, find the .htaccess file, add the following command in it (php_flag output_buffering on), and your problem will be solved.

Answer 1

Here's what you can do to troubleshoot and resolve this issue:

Check the specified lines: The warning message shows that the output begins on line 12 of /some/file.php, and you are on line 23 Try modifying the header information. Go to these lines and make sure there are no spaces, echo, print, HTML, or other output before the header() function call on line 23.

Remove any leading spaces: If any PHP files are included or required before the current file (e.g. using include or require), make sure that these files do not have any leading spaces before the opening <?php tag. Whitespace outside PHP tags is considered output and may cause this warning.

Avoid the closing PHP tag: In PHP files that contain only PHP code (no HTML), it is generally recommended to omit the closing PHP tag (?>) at the end of the file. This ensures there are no unexpected spaces or characters after the closing tag to avoid triggering the output.

Use output buffering: If there is some unavoidable output before the header() call (e.g. due to library dependencies or conditional statements), you can use output buffering to capture the output and prevent it from being sent to the browser immediately. Use ob_start() at the beginning of the PHP script to start the output buffering, and use ob_end_flush() or ob_end_clean() to end the output buffering before the header() call.

Example of using output buffering:

<?php 
ob_start(); // Start output buffering
// Code that generates output (if any)
// Now, set the headers
header('Location: new_page.php');
exit();
ob_end_flush(); // Send the output to the browser
?>