Header () is used to send custom HTTP messages

The header () is used to send custom HTTP messages. For more information on HTTP messages, please refer to. http/1.1 specification.

Note that the header () must be called before any actual output, whether it is an ordinary HTML tag, a blank line in the file, a space or a blank line in a php file, or a space. This is a very common mistake, if you have a space or a blank line before the header () is called through include,require, or its access to functions inside other files. If you are not invoking another file, just using a PHP or HTML file alone, there will be an output error before the header () is called.

<?php
/* This will give an error. Note the output
 * above, which is before the header() call */
header(‘Location: http://www.example.com/‘);
exit;
?>
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From for notes (Wiz)

Header () is used to send custom HTTP messages