What is cross-domain access? How to set access permissions using PHP

Cross-domain access problem is a common problem in front-end development. When we request resources from another different domain through Ajax or fetch in a web page or application, the problem of cross-domain access occurs. If we do not set relevant permissions, this kind of cross-domain access will not be allowed by the browser. This article will introduce how to set cross-domain access permissions using PHP.

1. What is cross-domain access?

Cross-domain access refers to a Web page in one domain accessing Web resources (such as scripts, style sheets, pictures, etc.) in another domain ). Cross-domain access involves the browser's security mechanism and is restricted for security reasons.

When we use Ajax or fetch in a Web page to request Web resources in another domain, cross-domain problems will occur. The browser will output an error message similar to "Access to XMLHttpRequest at 'http://abc.com/api/getdata' from origin 'http://xyz.com' has been blocked by CORS policy" on the console, that is, across The domain request was intercepted by the browser.

2. Why do you need to set cross-domain access permissions

In order to ensure the security of web applications, browsers restrict cross-domain requests. If we do not set relevant cross-domain access permissions, cross-domain requests will be prohibited by the browser and the corresponding data cannot be obtained. For some applications with separate front-end and back-end or applications that need to access API interfaces, the restriction of cross-domain requests will become a bottleneck and affect the normal operation of web applications.

3. How to set cross-domain access permissions in PHP

PHP is a web development language based on server-side scripting language. We can set cross-domain access permissions in PHP. Let's introduce how to set cross-domain access permissions in PHP.

1. Use the header() function to set cross-domain request headers

We can use the header() function in PHP to set cross-domain request headers. The so-called cross-domain request header is the "Access-Control-Allow-Origin" header in the HTTP protocol. Its function is to tell the browser whether the request is allowed cross-domain access.

The following is a sample code:

header('Access-Control-Allow-Origin: *');

This code is used to set the "Access-Control-Allow-Origin" header, and its parameter is "", which means that any domain is allowed to access the resource. Of course, we can also replace "" with the specified domain name, indicating that only specific domain names are allowed to access the resource.

2. Set other cross-domain request headers

In addition to the "Access-Control-Allow-Origin" header, there are some other cross-domain request headers, such as "Access-Control- Allow-Credentials", "Access-Control-Allow-Methods", "Access-Control-Allow-Headers", etc. You can use code similar to the following to set it:

header('Access-Control-Allow-Credentials: true');
header('Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS');
header('Access-Control-Allow-Headers: X-Requested-With, Content-Type, Authorization');

The meanings of these headers are:

• Access-Control-Allow-Credentials: Tell the browser whether to allow credentials such as cookies request to access the resource.
• Access-Control-Allow-Methods: Tell the browser the HTTP methods supported by this resource.
• Access-Control-Allow-Headers: Tell the browser the HTTP request headers supported by this resource.

3. Processing preflight requests

When the browser sends some HTTP requests, such as Socket.IO is actually a variant of HTTP, it can not only make simple requests ( GET, POST), and can also perform complex requests (PUT, DELETE, OPTIONS, etc.). In the case of complex requests, the browser will first send an OPTIONS request to ask the server whether to allow the cross-domain request. At this time, the server needs to return the corresponding response header. In this response header, you can set:

• Access-Control-Allow-Origin: Indicates the source that allows cross-domain requests (sometimes you can use * to represent all support).
• Access-Control-Allow-Headers: Indicates supported request headers.
• Access-Control-Allow-Methods: Indicates methods that support cross-domain requests.

The following is a sample code for processing preflight requests:

if ($_SERVER['REQUEST_METHOD'] == 'OPTIONS') {
    header('Access-Control-Allow-Origin: *');
    header('Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS');
    header('Access-Control-Allow-Headers: X-Requested-With, Content-Type, Authorization');
    exit;
}

This code is used to process OPTIONS requests, in which "Access-Control-Allow-Methods", "Access -Control-Allow-Headers" and other cross-domain request headers.

4. Summary

Setting cross-domain access permissions through PHP can solve cross-domain problems in front-end development. We can use the header() function in PHP to set cross-domain request headers, and we can also set other cross-domain request headers to meet business needs. When processing preflight requests, we need to return corresponding response headers to tell the browser whether the resource supports cross-origin requests. Through the above steps, we can effectively solve cross-domain problems and ensure the normal operation of web applications.

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