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Most PHP programmers would have come across this old chestnut before:
Cannot modify header information – headers already sent by page.php line 42

This can happen when you try doing a HTTP redirect or send HTTP 404, as it can happen whenever you try to send headers once they’ve already been sent. When a user sees this, they are basically left high and dry, often on a blank page. The solution is simple:

Detect if Headers Have Already Been Sent

Whenever you call header(); , call headers_sent(); first!

Example:

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    if (!headers_sent()) {
        header('Location: ' . (!$absolute ? HOME_URL : "") . $page);
        die();
    } else {
        echo "\n\n<p>Couldn't redirect to " . htmlentities((!$absolute ? HOME_URL : "") . $page) . ", headers already sent.</p>\n\n";
        //do something here...
    }

Of course you should probably do thing useful

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This entry was posted on Wednesday, July 22nd, 2009 at 3:39 pm and is filed under PHP, Troubleshooting. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.