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In PHP programming, the header() function is used to send raw HTTP headers to the client, typically for redirection, setting content type, and other operations. However, during development, a common error may arise: headers already sent. This error typically occurs when you attempt to use the header() function after outputting content.
Today, we will explore the reasons behind this error and demonstrate how to resolve it using ob_start() through a practical case study.
In PHP, the header() function must be called before any content is sent to the page. If any output (such as HTML tags, whitespace, or error messages) is sent before calling header(), PHP will throw the headers already sent error. PHP attempts to send HTTP headers to the browser, but since content has already been sent, it cannot modify the headers, resulting in an error.
Unexpected spaces or newline characters: One of the most common causes is extra spaces or newline characters at the beginning or end of the file. Any output before the <?php tag or after the closing ?> tag will cause PHP to automatically send HTTP headers.
HTML or other output content: If the PHP script outputs HTML, echo statements, or any other non-PHP content before calling header(), it will trigger the headers already sent error.
One common solution is to use PHP's output buffering function ob_start() to prevent content from being output before calling header().
Suppose we have a simple PHP script that attempts to redirect to another page, but encounters the headers already sent error during execution.
Problematic Code:
<?php
// Assume there are unexpected spaces or newline characters at the start of the file
echo "This is some content before the header.";
<p>// Attempt redirection<br>
header("Location: <a rel="noopener" target="_new" class="" href="http://m66.net/anotherpage.php">http://m66.net/anotherpage.php</a>");<br>
exit();<br>
?>If you run this code, the browser might display the following error message:
Warning: Cannot modify header information - headers already sent by (output started at /path/to/script.php:2) in /path/to/script.php on line 6The issue occurs because the echo statement has already output content, causing the HTTP headers to be sent. Therefore, when the header() function is called, it cannot modify the headers.
We can use PHP's output buffering feature to resolve this issue. The ob_start() function enables output buffering, so even if there is output in the code, PHP will store the content in the buffer until the script finishes or ob_end_flush() is called.
Fixed Code:
<?php
// Enable output buffering
ob_start();
<p>// Assume there are unexpected spaces or newline characters at the start of the file<br>
echo "This is some content before the header.";</p>
<p>// Attempt redirection<br>
header("Location: <a rel="noopener" target="_new" class="" href="http://m66.net/anotherpage.php">http://m66.net/anotherpage.php</a>");<br>
exit();</p>
<p>// End output buffering and send all content<br>
ob_end_flush();<br>
?>In this revised code, ob_start() enables output buffering. Even though echo outputs content, it is not immediately sent to the browser; instead, it is stored in the buffer until the script finishes. When the header() function is called, PHP can still modify the HTTP headers, thus preventing the headers already sent error.
By using ob_start() and output buffering, we can prevent the "headers already sent" error when calling the header() function. This method is highly useful in practical development, especially when you cannot control other output content (such as files included or third-party libraries).
If you encounter this error, try adding ob_start() at the beginning of your code. It is a simple and effective solution.
