How to prevent header injection attacks in PHP?

@ryleigh 

Header injection attacks occur when an attacker is able to manipulate the HTTP headers that are sent by a web application. This can be done by injecting additional header fields or modifying existing ones in a way that allows the attacker to execute malicious code or steal sensitive information.

Here are some measures you can take to prevent header injection attacks in PHP:

1. Sanitize user input: Always sanitize user input, especially if it is used to construct HTTP headers. Use functions such as htmlspecialchars() and strip_tags() to remove any potentially malicious characters.
2. Validate user input: Validate user input to ensure that it conforms to the expected format. For example, if you expect an email address, use a regular expression or a validation library to ensure that the input is indeed an email address.
3. Use built-in PHP functions: Use built-in PHP functions to set headers instead of constructing them manually. For example, use header() instead of concatenating strings to create headers.
4. Use an output encoding library: Use an output encoding library such as OWASP's ESAPI to encode output before it is sent to the client. This can help prevent header injection attacks as well as other types of attacks such as cross-site scripting (XSS).
5. Use a web application firewall: Use a web application firewall (WAF) to help protect against header injection attacks. A WAF can detect and block malicious traffic before it reaches the application.

By implementing these measures, you can help prevent header injection attacks in your PHP application. However, it's important to keep in mind that security is an ongoing process, and it's important to stay up-to-date on the latest security best practices and vulnerabilities.