Basic crawler tutorial: Implement basic web crawler using PHP

With the continuous development and progress of Internet technology, people have more and more ways to obtain information. As one of the tools, web crawlers play an increasingly important role in helping people obtain information. A web crawler is an automated program that collects, obtains, analyzes and stores data from web pages on the Internet.

The implementation of web crawlers can be carried out through a variety of programming languages. Among them, PHP language is a language widely used in the field of network development. It has the characteristics of easy to learn, easy to use, and high development efficiency. Therefore, this article will use PHP language As an example, we will introduce how to use PHP to implement a basic web crawler.

1. Overview

You need to understand the following points when starting to learn PHP web crawlers:

1. The basic working principle of web crawlers: web crawlers request pages from the network information, obtain the response, and parse the response data for data capture.

2. Web crawler crawling process: The crawler needs to collect URLs to build a crawler queue, request URLs to obtain HTML pages, parse data in HTML pages, and store data.

3. Web crawler parsing method: After the web crawler obtains the HTML page, it needs to parse the data and store the data. The parsing methods include regular expressions, DOM, XPath, etc.

2. Build a crawler queue

The first step to implement a crawler is to build a crawler queue, that is, construct a list of URLs to be crawled. In PHP, we can use arrays to store these URLs, and then traverse the array to make requests to these URLs. For example:

$url_list = array(
    'https://www.example.com/page1.html',
    'https://www.example.com/page2.html',
    'https://www.example.com/page3.html'
);
foreach($url_list as $url){
    //请求该URL并进行解析数据的操作
}

3. Request the URL to get the HTML page

In PHP, we can use the cURL extension module to send HTTP requests. cURL is a client-side URL transfer library that supports multiple protocols, allowing PHP web scripts to send files and data to other servers. cURL provides several methods to simulate browser access. Commonly used request methods are GET, POST, PUT, COOKIE and other request methods.

The following is a sample code for using cURL to request a URL:

//初始化cURL
$ch = curl_init();
//设置URL和其他请求选项
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
//执行HTTP请求
$result = curl_exec($ch);
//关闭cURL资源
curl_close($ch);

In this code, we first use the curl_init() function to initialize cURL, and then use the curl_setopt() function to set the request options, where CURLOPT_URL Used to specify the URL of the request, CURLOPT_RETURNTRANSFER sets the request result to be returned as a string, and finally uses the curl_exec() function to make an HTTP request and obtain the request result. Use curl_close() function to close cURL resources.

4. Parse the data in the HTML page

After obtaining the HTML page, you need to extract useful information from it. There are many ways to parse HTML pages. Here we will parse them through DOM.

DOM parsing is a way of analyzing XML/HTML documents. In PHP, we can use the DOMDocument class to parse HTML pages. First, you need to instantiate the DOMDocument class, then use the loadHTML() method to load the HTML page into the parser, and finally use the getElementsByTagName() method to obtain the required element objects.

The following is a sample code for using DOM to parse an HTML page:

//实例化DOMDocument类
$dom = new DOMDocument();
//禁用HTML原始输入
$dom->preserveWhiteSpace = false;
//载入HTML页面
$dom->loadHTML($result);
//获取所需元素
$element = $dom->getElementsByTagName('div');

In this code, we first create a DOMDocument object, then use the preserveWhiteSpace attribute to remove whitespace characters in HTML, and then use loadHTML () method to load the HTML page, and finally use the getElementsByTagName() method to obtain the specified elements.

5. Store data

After extracting useful information, we need to store this information. In PHP, we can use MySQL database for data storage.

First, you need to use the mysqli_connect() function to connect to the MySQL database. Then use the mysqli_query() function to execute SQL statements to insert data into the database.

The following is a sample code for using MySQL database to store data:

//连接到MySQL数据库
$con = mysqli_connect('localhost', 'root', '', 'test');
//将数据插入到数据库中
mysqli_query($con, "INSERT INTO test (name, age) VALUES ('Tom', 20)");

In this code, we first use the mysqli_connect() function to connect to the MySQL database, and then use the mysqli_query() function to test Insert data into the table.

6. Summary

This article introduces the basic process of using PHP to implement web crawlers, including building a crawler queue, requesting URLs to obtain HTML pages, parsing data in HTML pages, and storing data. At the same time, this article is only a preliminary learning guide. There are many factors that need to be considered in actual development, such as data cleaning, anti-crawler mechanisms, etc. However, I believe that this article can provide a preliminary understanding of the implementation of PHP web crawlers and lay the foundation for further learning.

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