<?php
function pingDomain($domain){
    $starttime = microtime(true);
    $file      = @fsockopen ($domain, 80, $errno, $errstr, 10);
    $stoptime  = microtime(true);
    $status    = 0;

    if (!$file) $status = -1;  // Site is down
    else {
        fclose($file);
        $status = ($stoptime - $starttime) * 1000;
        $status = floor($status);
    }

    if ($status <> -1) {
        return true;
    }

    return false;

}
//llamando a la funcion
if (pingDomain('www.dominio.com')) {
    echo 'ON';
} else {
    echo 'OFF';
}