_ga Cookie值负责什么？

Question

使用PHP会话进行实验并显示cookie数组，但是在cookie中找到了_ga变量/值。哪个应用程序/谁在写我的Cookie？

我已经删除了代码。

$time = $_SERVER['REQUEST_TIME'];
echo "<PRE>";
echo session_id();
echo "<BR>cookie:<BR>";
print_r($_COOKIE);
echo "</PRE>";
if (isset($_SESSION['LAST_ACTIVITY'])){
    echo "session last activity check<BR>";
    $tt = $time - $_SESSION['LAST_ACTIVITY'];
    echo "seconds passed : $tt<BR>";
    if($tt> 60) {
        echo "EXPIRED...";
        session_unset();
        session_destroy();
        session_start();  
        $_SESSION['LAST_ACTIVITY'] = $time;
    }
} else {
    echo "expired altogether or it's been forever!<BR>";
    $_SESSION['LAST_ACTIVITY'] = $time;
}
echo "<BR>time:" . date('Y-m-d h:i:s',$time);

我只希望PHPSESSID变量，但是输出显示了另一个变量_ga：

l7bbchk6ae51u53a89aanvvi5d
cookie:
Array
(
    [_ga] => GA1.2.37759029.1552805943
    [PHPSESSID] => l7bbchk6ae51u53a89aanvvi5d
)
session last activity check
seconds passed : 3

time:2019-04-17 02:39:12