Detecting client browser script in PHP scripting language

<?php
function detect_browser()
{
    if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') == TRUE)
    {
        if(strpos($_SERVER['HTTP_USER_AGENT'], 'Opera') == FALSE)
        {
            $browser = 1;
        }
    }

    if(strpos($_SERVER['HTTP_USER_AGENT'], 'Opera') != FALSE)
    {
        $browser = 2;
    }

    if(strpos($_SERVER['HTTP_USER_AGENT'], 'U') != FALSE && strpos($_SERVER['HTTP_USER_AGENT'], 'Gecko') != FALSE)
    {
        $browser = 3;
    }

    if(strpos($_SERVER['HTTP_USER_AGENT'], 'U') != FALSE || strpos($_SERVER["HTTP_USER_AGENT"], 'Nav') != FALSE)
    {
        if(strpos($_SERVER['HTTP_USER_AGENT'], 'Opera') == FALSE)
        {
            if( strpos($_SERVER['HTTP_USER_AGENT'], 'US') == FALSE )
            {
                if(strpos($_SERVER['HTTP_USER_AGENT'], 'Gecko') == FALSE)
                {
                    if(strpos($_SERVER['HTTP_USER_AGENT'], 'Mozilla/5') == FALSE)
                    {
                        $browser = 4;
                    }
                }
            }
        }
    }

    switch($browser)
    {
        case 1:
            if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 4') != FALSE)
            {
                echo 'MSIE 4.<br>';
            }
            elseif(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 5') != FALSE)
            {
                echo 'MSIE 5.<br>';
            }
            elseif(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 6') != FALSE)
            {
                echo 'MSIE 6.<br>';
            }
            else
            {
                echo 'Not in MSIE 4, 5, 6.<br>';
            }
            break;
        case 2:
            if(strpos($_SERVER['HTTP_USER_AGENT'], 'Opera 5') != FALSE)
            {
                echo 'Opera 5.<br>';
            }
            elseif(strpos($_SERVER['HTTP_USER_AGENT'], 'Opera 6') != FALSE)
            {
                echo 'Opera 6.<br>';
            }
            elseif(strpos($_SERVER['HTTP_USER_AGENT'], 'Opera 7') != FALSE)
            {
                echo 'Opera 7.<br>';
            }
            else
            {
                echo 'Not in Opera 5, 6, 7.<br>';
            }
            break;
        case 3:
            echo '\'Netscape Gecko\' technology; Mozilla/5.0 compatible browser.<br>';
            break;
        case 4:
            echo 'Nescape browser, neither \'Gecko-powered\' nor \'Mozilla/5.0\' compatible.<br>';
            break;
        default:
            echo 'Not in MSIE, Opera, Netscape, Mozilla.<br>';
            break;
    }
}

detect_browser();
?>

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