Interval to check liveness.

check_liveness.php

<?php
$hostname = '192.168.0.100';
$port     = 80;
$timeout  = 10;
$sock     = fsockopen($hostname, $port, $errono, $errstr, $timeout);
if ($sock === false) {
  exit("{$errstr} ({$errono})\n");
}
echo "connect {$hostname}:{$port}\n";

$start = microtime(true);
echo "start: {$start}\n"; 

fgets($sock);                                                                                                                                              
$end = microtime(true);

fclose($sock);

echo "  end: {$end}\n";
echo " time: " . ($end - $start) . "\n";