You can't just have php run multiple times after a page has loaded. That PHP is executed one time when the page loads.
To do what you are attempting to do you should use some javascript and an ajax call.
$(function(){
function pingServer(){
$.post('/ping.php',function(data){
console.log('server is ok');
});
}
setInterval( pingServer, 4000 );
});
Also you may not need to ping the server every that often (every 100). Otherwise you may have issues.