A small PHP function to log a message to the HTTP header(s) X-Log-Message.

log2header.php

function __log2header($message)
{
    if (headers_sent())
    {
        return false;
    }

    // Escape newline (not allowed in headers)
    $message = str_replace("\n", "\\n", $message);
    
    if(mb_strlen($message) > 8000)
    {
        // Cut long messages after 8000 characters
        // (since headers shouldn't be much longer than that)
        $message = mb_substr($message, 0, 8000).'...';
    }

    header("X-Log-Message: $message", false);

    return true;
}

It seems current versions of Google Chrome and Internet Explorer show header values in UTF-8, while Firefox shows them in ISO 8859-1. This function assumes UTF-8.