PHP web crawler, data to json

I'm making a web crawler for my site but my code is sometimes giving an error: - Notice: trying to get property of non-object in C:...\index.php on line 25

(The index.php delivers al the variables to the pages.json file)

This is my code (index.php)

<?php

// This is our starting point. Change this to whatever URL you want.
$start = "test.html";

// Our 2 global arrays containing our links to be crawled.
$already_crawled = array();
$crawling = array();

function get_details($url) {

    // The array that we pass to stream_context_create() to modify our User Agent.
    $options = array('http'=>array('method'=>"GET", 'headers'=>"User-Agent: howBot/0.1\n"));
    // Create the stream context.
    $context = stream_context_create($options);
    // Create a new instance of PHP's DOMDocument class.
    $doc = new DOMDocument();
    // Use file_get_contents() to download the page, pass the output of file_get_contents()
    // to PHP's DOMDocument class.
    @$doc->loadHTML(@file_get_contents($url, false, $context));

    // Create an array of all of the title tags.
    $title = $doc->getElementsByTagName("title");
    // There should only be one <title> on each page, so our array should have only 1 element.
    $title = $title->item(0)->nodeValue;
    // Give $description and $keywords no value initially. We do this to prevent errors.
    $description = "";
    $keywords = "";
    // Create an array of all of the pages <meta> tags. There will probably be lots of these.
    $metas = $doc->getElementsByTagName("meta");
    // Loop through all of the <meta> tags we find.
    for ($i = 0; $i < $metas->length; $i++) {
        $meta = $metas->item($i);
        // Get the description and the keywords.
        if (strtolower($meta->getAttribute("name")) == "description")
            $description = $meta->getAttribute("content");
        if (strtolower($meta->getAttribute("name")) == "keywords")
            $keywords = $meta->getAttribute("content");

    }
    // Return our JSON string containing the title, description, keywords and URL.
    return '{ "Title": "'.str_replace("\n", "", $title).'", "Description": "'.str_replace("\n", "", $description).'", "Keywords": "'.str_replace("\n", "", $keywords).'", "URL": "'.$url.'"},';

}

function follow_links($url) {
    // Give our function access to our crawl arrays.
    global $already_crawled;
    global $crawling;
    // The array that we pass to stream_context_create() to modify our User Agent.
    $options = array('http'=>array('method'=>"GET", 'headers'=>"User-Agent: howBot/0.1\n"));
    // Create the stream context.
    $context = stream_context_create($options);
    // Create a new instance of PHP's DOMDocument class.
    $doc = new DOMDocument();
    // Use file_get_contents() to download the page, pass the output of file_get_contents()
    // to PHP's DOMDocument class.
    @$doc->loadHTML(@file_get_contents($url, false, $context));
    // Create an array of all of the links we find on the page.
    $linklist = $doc->getElementsByTagName("a");
    // Loop through all of the links we find.
    foreach ($linklist as $link) {
        $l =  $link->getAttribute("href");
        // Process all of the links we find. This is covered in part 2 and part 3 of the video series.
        if (substr($l, 0, 1) == "/" && substr($l, 0, 2) != "//") {
            $l = parse_url($url)["scheme"]."://".parse_url($url)["host"].$l;
        } else if (substr($l, 0, 2) == "//") {
            $l = parse_url($url)["scheme"].":".$l;
        } else if (substr($l, 0, 2) == "./") {
            $l = parse_url($url)["scheme"]."://".parse_url($url)["host"].dirname(parse_url($url)["path"]).substr($l, 1);
        } else if (substr($l, 0, 1) == "#") {
            $l = parse_url($url)["scheme"]."://".parse_url($url)["host"].parse_url($url)["path"].$l;
        } else if (substr($l, 0, 3) == "../") {
            $l = parse_url($url)["scheme"]."://".parse_url($url)["host"]."/".$l;
        } else if (substr($l, 0, 11) == "javascript:") {
            continue;
        } else if (substr($l, 0, 5) != "https" && substr($l, 0, 4) != "http") {
            $l = parse_url($url)["scheme"]."://".parse_url($url)["host"]."/".$l;
        }
        // If the link isn't already in our crawl array add it, otherwise ignore it.
        if (!in_array($l, $already_crawled)) {
                $already_crawled[] = $l;
                $crawling[] = $l;
                // Output the page title, descriptions, keywords and URL. This output is
                // piped off to an external file using the command line.
                echo get_details($l)."\n";
        }

    }
    // Remove an item from the array after we have crawled it.
    // This prevents infinitely crawling the same page.
    array_shift($crawling);
    // Follow each link in the crawling array.
    foreach ($crawling as $site) {
        follow_links($site);
    }

}
// Begin the crawling process by crawling the starting link first.
follow_links($start);

My (test.html)

<a href="https://www.hln.be/">HLN</a>
<a href="//www.google.be/">Go to google</a>
<a href="//www.youtube.com/">Youtube</a>

I hope you guys can help me.

Greetings Wannes