How to implement a reliable web crawler with PHP and capture effective information

How PHP implements reliable web crawlers and captures effective information

With the development of the Internet and the increasing amount of data, the demand for web crawlers has become increasingly prosperous. Crawlers can automatically collect, extract, process and store large-scale data from the Internet, providing foundation and support for applications in all walks of life. This article will introduce how to use PHP to implement a reliable web crawler and capture effective information.

1. Principle of crawler

Web crawler, also known as web spider, web robot, web harvester, auto indexer or A spider program is a program that can automatically browse, index, and crawl various information on the Internet. The principle is to send a request to the target website through the HTTP protocol, parse the HTML content and metadata in the data returned by the website, extract the target information and store it. Implementing a web crawler requires the following elements:

1. Basic knowledge of HTTP requests and responses

1) HTTP request: The HTTP protocol is one of the most widely used protocols on the Internet. , the client requests content from the server through HTTP requests. An HTTP request consists of HTTP method, request resource identifier, protocol version, request header and request body.

2) HTTP response: HTTP response is the server's reply to the request. It consists of a status line (status code and status phrase), response headers, and a response body, where the response body is the content of the requested resource.

2. HTML document parsing and processing technology

HTML is a markup language used to design web pages, using English tags to embed text, images, audio and other elements into in the web page. Therefore, in the process of implementing a web crawler, you need to be able to understand the HTML document structure, tag semantics, and other metadata.

3. Data storage and management capabilities

The captured data needs to be structured and stored in a database or file to realize data visualization and query. This requires an understanding of database structure and SQL language.

2. PHP crawler implementation

In PHP, you can use a third-party crawler framework or implement the crawler yourself. Here are two commonly used methods:

1. Use a third-party crawler framework

1) Goutte

Goutte is a web crawler and web extraction component for PHP 5.3 . It can simulate a real browser and provide jQuery-like operation API to facilitate data extraction and operation. It also supports functions such as cookies and HTTP proxy. Due to its ease of use, support and flexibility, more and more developers have chosen this library to build their web crawlers in recent years.

2) PHP-Webdriver

PHP-Webdriver is a Selenium client library in PHP that allows PHP code to communicate with Selenium WebDriver (or other WebDriver) and control the browser's running process. This is more suitable for examples where you need to crawl data from dynamic pages. For example: Table rendered using JS, etc.

Example:

Install Goutte:

composer require fabpot/goutte:^3.2

Use Goutte:

use GoutteClient;

$client = new Client();
$crawler = $client->request('GET', 'https://www.baidu.com/');
$form = $crawler->filter('#form')->form();
$crawler = $client->submit($form, array('q' => 'search'));

2. Handwritten PHP crawler

The advantage of handwritten crawler is It has a better understanding of the behavior of crawlers, so it can make more detailed and personalized configurations. At this point it can be divided into three parts: requesting the page, parsing the page and storing data.

1) Request the page

Use PHP's CURL extension to simulate an HTTP request to obtain the page content. CURL can send requests based on the HTTP protocol and return an HTTP response for a given URL.

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HEADER, false);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
$content = curl_exec($ch);

2) Parse the page

Use PHP's DOMDocument class to parse the HTML page to construct the DOM tree, and use XPath technology (query language for XML and HTML documents) to extract the page content through rules .

$dom = new DOMDocument();
@$dom->loadHTML($content);
$xPath = new DOMXPath($dom);
$items = $xpath->query("//div[@class='items']//h2//a");
foreach ($items as $item) {
  $title = trim($item->childNodes->item(0)->nodeValue);
  $link = $item->attributes->getNamedItem("href")->nodeValue;
  $data[] = array(
    "title" => $title,
    "link" => $link
  );
}

3) Store data

Store the data captured from the page into a database or file. Databases such as MySQL or MongoDb can be used to store data.

$mysql = new mysqli('localhost', 'username', 'password', 'db');
foreach ($data as $item) {
  $title = $mysql->real_escape_string($item['title']);
  $link = $mysql->real_escape_string($item['link']);
  $sql = "INSERT INTO table(title,link) VALUES ('$title','$link')";
  if ($mysql->query($sql) === true) {
    $inserted[] = $item;
  }
}

3. Points to note during the crawling process

1. Dealing with website anti-crawlers

In order to limit the behavior of crawlers, some websites will use some technologies to Block crawlers, such as using verification codes, blocking IP, speed limiting, etc. In order to avoid being restricted by anti-crawler policies, you need to circumvent restrictions based on the website's anti-crawler technology.

2. Use Proxy Reasonably

During the crawling process, there may be cases where the IP is blocked by the website. A simple method is to use a proxy IP to access the website. At the same time, you can use a proxy IP pool to reduce the risk of IP being blocked.

3. Control request frequency

Frequent requests may cause disturbances to the anti-crawler mechanism, so the crawler request speed needs to be appropriately controlled. Implementation methods include: using the sleep method to control the time interval between two requests; using the message queue to control the number of messages sent within a specified period of time; spreading requests over multiple time periods to avoid frequent requests in a short period of time.

4. Conclusion

Web crawler is a very useful and practical technology that can help us quickly obtain and organize large amounts of data. This article introduces the method of implementing reliable web crawlers through PHP, and understands the basic principles of crawlers, related frameworks and the process of manually writing crawlers, as well as the points to pay attention to during the crawling process. I hope this article can help you with practical applications when writing web crawlers in the future.

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