Execution Flow

Survey application: Using a browser or an automated tool, an adversary follows all public links on a web site. They record all the links they find.

Techniques
Use a spidering tool to follow and record all links. Make special note of any links that include parameters in the URL.
Use a proxy tool to record all links visited during a manual traversal of the web application. Make special note of any links that include parameters in the URL. Manual traversal of this type is frequently necessary to identify forms that are GET method forms rather than POST forms.
Use a browser to manually explore the website and analyze how it is constructed. Many browser's plugins are available to facilitate the analysis or automate the URL discovery.

Attempt variations on input parameters: The attack variants make use of a remotely available PHP script that generates a uniquely identifiable output when executed on the target application server. Possibly using an automated tool, an adversary requests variations on the inputs they surveyed before. They send parameters that include variations of payloads which include a reference to the remote PHP script. They record all the responses from the server that include the output of the execution of remote PHP script.

Techniques
Use a list of probe strings to inject in parameters of known URLs. The probe strings are variants of PHP remote file inclusion payloads which include a reference to the adversary controlled remote PHP script.
Use a proxy tool to record results of manual input of remote file inclusion probes in known URLs.

Run arbitrary server-side code: As the adversary succeeds in exploiting the vulnerability, they are able to execute server-side code within the application. The malicious code has virtual access to the same resources as the targeted application. Note that the adversary might include shell code in their script and execute commands on the server under the same privileges as the PHP runtime is running with.

Techniques
Develop malicious PHP script that is injected through vectors identified during the Experiment Phase and executed by the application server to execute a custom PHP script.

PHP Remote File Inclusion