In PHP, how can you obtain information such as the client's IP address and the URL of the previous referring page?

Introduction / Context:
Many PHP applications need access to request metadata, such as the IP address of the client and the URL of the page that referred the user to the current script. This information is often used for logging, analytics, basic security checks, or personalization. PHP exposes HTTP request and server information through the $_SERVER superglobal array, making it straightforward to access client IP and referrer details when they are available.

Given Data / Assumptions:

• We are handling an HTTP request in a PHP script running on a web server.
• The web server provides environment variables like REMOTE_ADDR and HTTP_REFERER.
• PHP populates the $_SERVER superglobal array with these values when the request is processed.
• The question asks how to obtain the client IP and previous reference (referrer) page.

Concept / Approach:
The $_SERVER array contains many predefined indices that describe the current request and server environment. To get the apparent client IP address, you normally read $_SERVER[\'REMOTE_ADDR\'], which is set by the web server. To get the referring page, you can read $_SERVER[\'HTTP_REFERER\'], which is based on the HTTP Referer header sent by the client. However, you should remember that HTTP_REFERER can be missing or spoofed, so it should not be fully trusted for security sensitive decisions. Nevertheless, in terms of PHP syntax, using $_SERVER is the correct way to retrieve this information.

Step-by-Step Solution:
Step 1: Recognize that PHP stores server and request metadata in the $_SERVER superglobal array. Step 2: Access the client IP address using $ip = $_SERVER['REMOTE_ADDR']; which typically returns the IP of the immediate client seen by the server. Step 3: Access the referring page URL, if provided, using $referrer = isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : '';. Step 4: Optionally, consider proxy or load balancer headers such as HTTP_X_FORWARDED_FOR in environments where REMOTE_ADDR may contain a proxy IP instead of the original client. Step 5: Use these values for logging, analytics, or user experience tweaks, keeping in mind that they are not guaranteed to be present or accurate.

Verification / Alternative check:
You can test this by creating a PHP script that echoes $_SERVER[\'REMOTE_ADDR\'] and $_SERVER[\'HTTP_REFERER\']. When accessing the script directly from a browser, you will see your client IP in REMOTE_ADDR. If you navigate to the script using a link from another page, the referer header may be set and visible in HTTP_REFERER. Inspecting network requests in browser developer tools will show matching header values, confirming that $_SERVER is the right place to read this information.

Why Other Options Are Wrong:
Option B is incorrect because $_COOKIE stores cookies sent by the client, not automatically populated IP and referrer values. Option C is wrong; phpinfo() prints a configuration report and is not used to directly retrieve client IP or referrer programmatically. Option D is also incorrect because PHP does not create a special ip_referrer.log file automatically; log files are managed by server configuration or custom application code.

Common Pitfalls:
A common pitfall is trusting HTTP_REFERER for security decisions such as access control or CSRF prevention; it can be missing or easily forged. Another pitfall is assuming REMOTE_ADDR always contains the end user IP even behind proxies or load balancers. In such environments, you may need to look at X-Forwarded-For or similar headers, while still validating and sanitizing them. Always treat these values as untrusted input and use them carefully.

Final Answer:
In PHP you typically obtain the client IP and referrer by reading $_SERVER[\'REMOTE_ADDR\'] for the client IP address and $_SERVER[\'HTTP_REFERER\'] for the referring page URL, both provided via the $_SERVER superglobal array.