PHP crawler practice: obtaining web page source code and content analysis

PHP crawler is a program that automatically obtains web page information. It can obtain web page code, crawl data and store it locally or in a database. Using crawlers can quickly obtain large amounts of data, providing great help for subsequent data analysis and processing. This article will introduce how to use PHP to implement a simple crawler to obtain web page source code and content analysis.

1. Obtain the web page source code

Before we begin, we should first understand the basic structure of the HTTP protocol and HTML. HTTP is the abbreviation of HyperText Transfer Protocol, which is a protocol used to transfer web pages and data. Web pages are generally written in HTML, a markup language used to describe the structure and content of web pages. Now that we understand these basics, we can start writing our PHP crawler.

First, we need to provide a URL to specify the web page we want to crawl. In PHP, we can use the file_get_contents function to obtain the source code of the web page. This function will read the entire content of the web page corresponding to the specified URL in the form of a string. For example:

$url = "https://www.example.com";
$html = file_get_contents($url);

In this way, the read web page source code will be stored in the $html variable. It should be noted that the file_get_contents function can only read remote files. If you need to read local files, you should use the file function.

2. Content Analysis

After obtaining the source code of the web page, we need to extract the data we need from it. Generally speaking, web pages are composed of HTML code. We need to parse the HTML code to obtain the data we need.

In PHP, there are many HTML parsing libraries to choose from, such as DOMDocument, Simple HTML DOM, etc. Here we introduce a more commonly used parsing library-Simple HTML DOM. The Simple HTML DOM library can be used to parse and manipulate HTML documents. It provides a simple and easy-to-use interface to easily extract data from HTML.

Before using the Simple HTML DOM library, we need to download and import the library file first. The download address is https://sourceforge.net/projects/simplehtmldom/, and you can unzip it after downloading.

The steps to use the Simple HTML DOM library are as follows:

1. Introduce the library file:

include("simple_html_dom.php");

2. Create a new Simple HTML DOM object:

$html = new simple_html_dom();

3. Pass the web page source code we obtained earlier into the object:

$html->load($html);

4. Use the selector to select the elements we need:

$element = $html->find("tagName");

where tagName is the tag name of the element that needs to be selected. For example, if we need to get all a tags, we can use $html->find("a").

5. Use attributes to get the value of an element:

$value = $element->attributeName;

where attributeName is the attribute name that needs to be obtained. For example, if we need to obtain the href attribute of a tag, we can use $element->href.

6. Finally, don’t forget to destroy the Simple HTML DOM object:

$html->clear();
unset($html);

For example, if we need to get all the links from the Baidu homepage, we can do it as follows:

load($html);

$links = $dom->find("a");

foreach ($links as $link) {
    echo $link->href . "
";
}

$dom->clear();
unset($dom);

Through the above code, we can get all the links in Baidu homepage.

3. Summary

This article introduces how to use PHP to write a crawler, including obtaining web page source code and content analysis. You can use the file_get_contents function to obtain web page source code, and you can use the Simple HTML DOM library to parse HTML code. Readers can change and extend it according to their own needs and implement their own PHP crawler program.

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