How to resolve "Cannot Modify header information - headers already sent by" error in PHP?

Question

Because when we make use of the functions header() and setcookie() error occurs:

Warning: Cannot Modify header information - headers already sent by (output Started at /teste.php:10) in /php test. on line 21

Why does this mistake happen? How to solve?

Answer 1

In fact, it is not a mistake. It is a warning(Warning).

This is done when your browser requests the web server. With the response from the web server. It gives you a response header(header). This header contains information about page encoding, page size, cache duration time, last update time, and everything relevant about a web page to a browser.

When you read "headers already sent" means the server has already sent the header and AFTER this submission you are trying to change some information that should be sent in the header.

But if you’re not manipulating anything that comes in header. You have done nothing and are getting this error.

In PHP the header starts uploading as soon as you enter the first HTML character. Either outside of PHP or inside PHP code with an echo or print().

<?php
  $numero_1 = 5;
  echo $numero_1;
?>

Everything outside of PHP code is HTML, a space on line 1 before the opening of PHP code would be the reason for a response to the client. Any function, session, cookie would cause error.

To fix this warning. You would have to put all the code that works with Session, cookie, redirects, etc... before any HTML character. No trying to set/create a cookie after sending a "Hello world" message to your browser.

If you need to set/create a cookie before sending a "Hello world" message or anything, rethink what you are doing.

Answer 2

It depends, if you are editing with Dreamweaver it adds to your files at the time of saving an option called GOOD ENCODING (something of the type), this prevents your scripts in PHP run properly mainly on linux servers, windows works normally. Otherwise, see if php checks like:

<?php
if(isset($_POST['teste'])){
  header("Location: index.php");
  // é essencial utilizar o exit() após um redireccionamento com header() para evitar possíveis erros
  exit();
}
?>

And other checks are found in the page header, that is, the lines that do a check/redirect should always be above the code, always with the tags at the beginning of the HTML page. Forms, text, etc., can never be above php when redirects are made. Always above. Post your code to us that makes it easier to help.