How can PHP prevent XSS attacks?

In order to prevent XSS (cross-site scripting) attacks, you can follow these steps:

1. Input filtering: Filtering and validating user input data can be achieved by using PHP functions such as htmlspecialchars to escape special characters, or by using regular expressions to match specific input formats, in order to ensure that the data input does not contain any malicious script tags.
2. Output Encoding: When outputting data to an HTML page, use the appropriate encoding method, such as the htmlspecialchars function, to escape special characters into HTML entities in order to prevent malicious scripts from being executed.
3. Content Security Policy (CSP): Use CSP to restrict the sources of executable scripts on web pages, allowing script files to be loaded only from specified domain names to prevent malicious script injection. CSP can be enabled by setting HTTP headers or adding a tag.
4. Validate user input: Before processing user input data, perform rigorous input validation, such as checking if the input content meets the expected format and length, rejecting any illegal input.
5. Use secure database operations: utilize prepared statements or parameterized queries to avoid directly concatenating user input in SQL queries, in order to prevent SQL injection attacks.
6. Enable the HTTP-only flag: For cookies containing sensitive information, enable the HTTP-only flag to prevent JavaScript from accessing and modifying the cookie values using document.cookie.
7. Regularly update and upgrade: Keep PHP versions and related software libraries up to date to ensure you have the latest security patches and fixes, reducing the risk of known vulnerabilities.

The above steps do not guarantee absolute prevention of XSS attacks, so it is recommended to combine multiple defense measures and stay informed about the latest security threats and vulnerabilities.