PHP Cron Job with exec() function - php

Question

I am trying to program a Cron Job that fetches some data from multiple websites. The script uses phantomjs that runs a js file responsible for saving each website's content to a file locally.
My cron job command:
/usr/local/bin/php /home/user/public_html/fetch.php >> /home/user/public_html/fetch-log.txt
fetch.php
ini_set('max_execution_time', 300);
require_once 'Utils.php';
$sources = Utils::get_sources();
foreach ($sources as $source) {
$cmd = "/home/user/bin/phantomjs get-website.js '$source->url' $source->id";
exec($cmd);
}
echo date("Y-m-d H:i:s") . ">> Done fetching sources\n";
get-website.js
var system = require('system');
var webPage = require('webpage');
var fs = require('fs');
var page = webPage.create();
var url = system.args[1];
var file = system.args[2];
var requestsArray = [];
page.onResourceRequested = function (requestData, networkRequest) {
requestsArray.push(requestData.id);
};
page.onResourceReceived = function (response) {
var index = requestsArray.indexOf(response.id);
requestsArray.splice(index, 1);
};
page.open(url, function (status) {
var interval = setInterval(function () {
if (requestsArray.length === 0) {
clearInterval(interval);
try {
fs.write("/home/user/public_html/rss_contents/" + file + ".txt", page.content, 'w');
} catch (e) {
console.log(e);
}
phantom.exit();
}
}, 500);
});
The script works normally when running from browser, and the cron job also works normally when run with a simple echo("done"); php script, but when I try to run the cron job with the fetch.php script, the last line is echoed to the file, but nothing has happen.
I am using shared hosting.
EDIT:
When trying to run phantomjs from ssh console directly, I get this error
2019-06-27T11:34:12 [WARNING] QThread::start: Thread creation error:
Resource temporarily unavailable
And the console just hangs without any response.
I couldn't find any posts talking about this warning on Google.