Log poisoning is a type of LFI vulnerability that allows an attacker to tamper with the log files (apache.log, ssh.log, mail.log) by injecting php code into it. In simple terms in works by:
(PHP) reads the log file.
(PHP) detects the (PHP) code in the log file.
(PHP) will parse the (PHP) code and parse its output to the user's browser, displaying the (PHP) info or running the (PHP) code.