php网页爬虫 能进行广度优先爬取吗

<?php
class WebCrawler {
    private $visitedUrls = [];
    private $urlQueue = [];
    private $maxDepth;

    public function __construct($startUrl, $maxDepth) {
        $this->urlQueue[] = $startUrl;
        $this->maxDepth = $maxDepth;
    }

    public function start() {
        while (!empty($this->urlQueue)) {
            $currentUrl = array_shift($this->urlQueue);
            if (in_array($currentUrl, $this->visitedUrls)) {
                continue;
            }

            $this->visitedUrls[] = $currentUrl;
            echo "Crawling: " . $currentUrl . "\n";

            $this->fetchUrls($currentUrl);
        }
    }

    private function fetchUrls($url) {
        $html = file_get_contents($url);
        $links = $this->parseLinks($html);

        foreach ($links as $link) {
            $fullUrl = $this->buildFullUrl($url, $link);
            if (!$this->isVisited($fullUrl) && $this->isValidUrl($fullUrl) && $this->isWithinDepth($fullUrl)) {
                $this->urlQueue[] = $fullUrl;
            }
        }
    }

    private function parseLinks($html) {
        // 使用正则表达式或其他方法解析HTML并提取链接
        // 示例代码省略了具体的解析逻辑
    }

    private function isVisited($url) {
        return in_array($url, $this->visitedUrls);
    }

    private function isValidUrl($url) {
        // 检查URL是否有效
        // 示例代码省略了具体的验证逻辑
    }

    private function isWithinDepth($url) {
        $parsedUrl = parse_url($url);
        $currentDepth = count(explode('/', $parsedUrl['path']));
        return $currentDepth <= $this->maxDepth;
    }

    private function buildFullUrl($base, $relative) {
        $parsedBase = parse_url($base);
        $relativeParts = explode('/', trim($relative, '/'));
        $fullParts = array_merge([$parsedBase['host']], $relativeParts);
        return $parsedBase['scheme'] . '://' . implode('/', $fullParts);
    }
}

// 使用示例
$crawler = new WebCrawler('https://example.com', 2);
$crawler->start();
?>