PHP-based crawler implementation: how to combat anti-crawler strategies

With the continuous development and popularization of the Internet, the demand for crawling website data is gradually increasing. In order to meet this demand, crawler technology came into being. As a popular development language, PHP is also widely used in crawler development. However, some websites adopt anti-crawler strategies in order to protect their data and resources from being easily crawled. So, how to combat these anti-crawler strategies in PHP crawler development? Let’s find out below.

1. Pre-requisite skills

If you want to develop an efficient crawler program, you need to have the following skills:

1. Basic HTML knowledge: including HTML structure , elements, tags, etc.
2. Familiar with the HTTP protocol: including request methods, status codes, message headers, response messages, etc.
3. Data analysis capabilities: Analyze the HTML structure, CSS styles, JavaScript code, etc. of the target website.
4. Certain programming experience: Familiar with the use of PHP and Python programming languages.

If you lack these basic skills, it is recommended to do basic learning first.

2. Crawl strategy

Before you start writing a crawler program, you need to understand the mechanism and anti-crawler strategy of the target website.

1. robots.txt Rules

robots.txt is a standard used by site administrators to tell crawlers which pages can and cannot be accessed. Please note that compliance with robots.txt rules is the first requirement for a crawler to be a legal crawler. If a robots.txt file is obtained, please check it first and crawl it according to its rules.

2. Request frequency

Many websites will limit access frequency to prevent crawlers from accessing too frequently. If you encounter this situation, you may consider adopting the following strategy:

• Request again after taking a break. You can use the sleep() function to wait for a period of time before making the request again.
• Parallel requests. You can use multiple processes or threads to send requests to improve efficiency.
• Simulate browser behavior. Simulating browser behavior is a good approach because it is difficult for the server hosting the website to tell whether your program is accessing the web page as a human.

3. Request header

Many websites use the request header information to determine whether to accept requests from crawlers. It is important to include the User-Agent information in the request header because this is important information sent by the browser. In addition, in order to better simulate user behavior, you may also need to add some other information to the request header, such as Referer, Cookie, etc.

4. Verification Code

Today, in order to deal with crawlers, many websites will add verification codes when users interact to distinguish machines from humans. If you encounter a website that requires you to enter a verification code to get data, you can choose the following solution:

• Automatically recognize the verification code, but this is not a feasible solution unless you have some excellent Third-party verification code solving tool.
• Manual solution. After reading the analysis page, you can manually enter the verification code and continue running your crawler. Although this solution is more cumbersome, it is feasible in harsh situations.

3. Code Implementation

When developing PHP crawlers, you need to use the following technologies:

1. Use cURL extension library

cURL is a powerful extension that enables your PHP scripts to interact with URLs. Using the cURL library, you can:

• Send GET and POST requests
• Customize HTTP request headers
• Send Cookies
• Use SSL and HTTP Authentication

It is one of the necessary technologies for executing crawlers. You can use cURL like this:

// 创建 cURL 句柄
$curl = curl_init(); 

// 设置 URL 和其他属性
curl_setopt($curl, CURLOPT_URL, "http://www.example.com/");
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HEADER, false);

// 发送请求并获取响应
$response = curl_exec($curl); 

// 关闭 cURL 句柄
curl_close($curl);

2. Using regular expressions

When crawling specific content, you may need to extract data from the HTML page. PHP has built-in support for regular expressions, and you can use regular expressions to achieve this functionality.

Suppose we need to extract the text in all title tags <h1> from an HTML page. You can achieve this by:

$html = ".....";
$pattern = '/<h1>(.*?)</h1>/s'; // 匹配所有 h1 标签里的内容
preg_match_all($pattern, $html, $matches);

3. Using PHP Simple HTML DOM Parser

PHP Simple HTML DOM Parser is a simple and easy-to-use PHP library that uses something like jQuery Selector syntax to select elements in an HTML document. You can use it to:

• Parse HTML pages and get elements
• Simulate clicks and submit forms
• Search for elements

Installation PHP Simple HTML DOM Parser is very simple and you can install it through Composer.

4. Use a proxy

Using a proxy is a very effective anti-anti-crawler strategy. You can spread your traffic across multiple IP addresses to avoid being rejected by the server or generating excessive traffic. Therefore, using a proxy allows you to perform your crawling tasks more safely.

Finally, no matter which strategy you adopt, you need to comply with relevant regulations, protocols and specifications in crawler development. It is important not to use crawlers to violate website confidentiality or obtain trade secrets. If you wish to use a crawler to collect data, make sure that the information you obtain is legal.

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