<?php
error_reporting(-1);
ini_set('display_errors', true);
stream_context_set_default(
    array(
        'http' => array(
            'method' => 'HEAD'
        )
    )
);

$links = [];
$checked = [];
loadRecursive(getMainUrl(), $links, $checked);


function getMainUrl()
{
    return 'http://theory.phphtml.net';
}

function loadRecursive1($url, array &$found, array &$checked)
{

    echo "Parsing " . $url . " <br/>";
    $checked[] = $url;
    $html = loadHTML($url);
    $links = findLinks($html);
    $counter = 0;
    foreach ($links as $link) {

        $link = getMainUrl() . $link;
        $linkKey = md5($link);
        if ($link === $url) {
            continue;
        }

        $found[$linkKey] = $link;
        $counter++;

        //  loadRecursive1($link, $found, $checked);

    }
    echo "Found " . $counter . " links on " . $url . "<hr/>";
}


function loadRecursive($url, array &$found, array &$checked)
{

    echo "Parsing " . $url . " <br/>";
    $checked[] = $url;

    $html = loadHTML($url);
    $links = findLinks($html);
    $counter = 0;
    foreach ($links as $link) {

        $link = getMainUrl() . $link;
        $linkKey = md5($link);
        if ($link === $url) {
            continue;
        }

        $found[$linkKey] = $link;
        $counter++;
    }
    foreach ($found as $link) {
        loadRecursive1($link, $found, $checked);
    }

    echo "Found " . $counter . " links on " . $url . "<hr/>";
}

function urlIsOk($url)
{

    $headers = @get_headers($url, 1);
    if (!$headers) {
        return false;
    }
    if (!preg_match("~(\\d{3})~", $headers[0], $matches)) {
        return false;
    }
    $statusCode = (int)$matches[0];
    if ($statusCode < 200 || $statusCode > 400) {
        return false;
    }
    if ($headers['Content-Type'] !== 'text/html') {
        return false;
    }

    return true;
}

function loadHTML($url)
{
    if (!urlIsOk($url)) {
        return '';
    }
    $curl = curl_init();
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, 0);
    $result = curl_exec($curl);
    curl_close($curl);
    if ($result === false) {
        //echo 'broken url';
        return '';
    }

    return (string)$result;
}

function findLinks($html)
{
    $domDocument = new DOMDocument();
    @$domDocument->loadHTML($html);

    $xpath = new DOMXPath($domDocument);

    /**
     * @var DOMNodeList $elements
     */
    $elements = $xpath->query('//a[@href]'); //найдти все елементы которыи имеют href аттрибут
    $links = [];

    foreach ($elements as $element) {
        $href = $element->getAttribute('href');
        if ($href === '') {
            continue;
        }
        $isAbsolute = strpos($href, 'http') !== false;
        $isAnchor = strpos($href, '#') !== false;
        if ($isAbsolute || $isAnchor) {
            continue;
        }
        $links[] = $href;
    }

    return array_unique($links);
}