put this in a file called common.
Class App_Util {
    function is_old_browser() {
        $old = 0;

        if (!empty($_SERVER['HTTP_USER_AGENT'])) {
            // IE <= 7
            // User Agent: Opera/9.80 (Windows NT 6.1; U; en) Presto/2.10.229 Version/11.61
            if (preg_match('#msie\s+(\d+)\.(\d+)#si', $_SERVER['HTTP_USER_AGENT'], $matches)) {
                if ($matches[1] <= 7) {
                    $old = 1;
                }
            }
            // Firefox <= 7
            // User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0.2) Gecko/20100101 Firefox/10.0.2
            elseif (preg_match('#Firefox/(\d+)\.(\d+)[\.\d]+#si', $_SERVER['HTTP_USER_AGENT'], $matches)) {
                if ($matches[1] <= 7) {
                    $old = 1;
                }
            }
            // Safari < 5
            // User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/534.52.7 (KHTML, like Gecko) Version/5.1.2 Safari/534.52.7
            elseif (preg_match('#Version/(\d+)[\.\d]+ Safari/[\.\d]+#si', $_SERVER['HTTP_USER_AGENT'], $matches)) {
                if ($matches[1] < 5) {
                    $old = 1;
                }
            }
            // opera < 11
            // User Agent: Opera/9.80 (Windows NT 6.1; U; en) Presto/2.10.229 Version/11.61
            elseif (preg_match('#Opera/[\.\d]+.*?Version/(\d+)[\.\d]+#si', $_SERVER['HTTP_USER_AGENT'], $matches)) {
                if ($matches[1] < 11) {
                    $old = 1;
                }
            }
        }

        return $old;
    }
}

How to perform the check
file: block_old_browser.php

<?php if (App_Util::is_old_browser()) : ?>
    <div class="app_old_browser_notice">
        <h2>Old Browser</h2>
        <p>Please consider upgrading your browser. Doing so will improve experience with the site. <br/>
        Please click one of them below.</p>
        <ul>
            <li><a href="http://www.mozilla.org/en-US/firefox/fx/" target="_blank">Mozilla Firefox</a></li>
            <li><a href="https://www.google.com/chrome" target="_blank">Google Chrome</a></li>
            <li><a href="http://www.apple.com/safari/" target="_blank">Safari</a></li>
            <li><a href="http://www.opera.com/download/" target="_blank">Opera</a></li>
            <li><a href="http://www.microsoft.com/windows/ie/" target="_blank">Internet Explorer (Windows)</a></li>
        </ul>
    </div>
<?php endif; ?>

Include the file above in all the files that you need to the noticed displayed.
<?php include('block_old_browser.php'); ?>

css styles
<style>
.app_old_browser_notice {
    background-color:#ff0000;
    color: white;
    margin:20px;
    padding:10px;
}

.app_old_browser_notice a {
    color: white;
}
</style>