Running EvPeriodic with a loop

I am trying to run EvPeridoc to log data periodically (every 15 minutes, i.e 900 seconds) to a file.

<?php
$tolog = '';
$w = new EvPeriodic(fmod(Ev::now(), 900), 900, NULL, function($w, $revents)
{
    file_put_contents("log.txt", $tolog . PHP_EOL, FILE_APPEND | LOCK_EX);
});
Ev::run(Ev::RUN_NOWAIT);
echo "Started periodic @ " . date("Y-m-d h:i:sa") . PHP_EOL;
for (;;) {
    $tolog = "Update @ " . date("Y-m-d h:i:sa");
}

The periodic never fires but the app goes into the for loop. If I change the line Ev::run(Ev::RUN_NOWAIT); to Ev::run(); then the script never reaches the loop.

So the question is how do I pipe this code to be able to fire logging routine every 15 mins?

EDIT: Just to clarify - the infinite loop at the end of this script generates the data to be logged (and assigns the value to the $tolog gloabl variable). What I'd like EvPeriodic to do is pick up that (global) variable every 15 minutes and append it to a file.

As per comments to the question, it is not possible to use both an infinite loop (which in my production app is a streaming http loop) and Evperiodic. I worked around this by using IPC and ended up by having to run the EvPeriodic code in a separate script.

Main app

<?php
$socket  = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
socket_bind($socket, '127.0.0.1', 15000);
socket_listen($socket);
socket_set_nonblock($socket);
$tolog= '';
for (;;){
        $tolog = "Update @ " . date("Y-m-d h:i:sa");
        if (($newc = socket_accept($socket)) !== false) {
                socket_write($newc, $tolog, strlen($tolog));
                socket_close($newc);
        }
}

The 'client' app with EvPeriodic polling the main app

<?php
$w = new EvPeriodic(fmod(Ev::now(), 15), 15, NULL, function($w, $revents){
        $sock = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
        socket_connect($sock, '127.0.0.1', 15000);
        $line = trim(socket_read($sock, 1024));
        file_put_contents("/var/log/cohocompanies.log", $line . PHP_EOL, FILE_APPEND | LOCK_EX);
        socket_close($sock);
});
Ev::run();