PHP Object Injection is an application level vulnerability that could allow an attacker to perform different kinds of malicious attacks, such as Code Injection, SQL Injection, Path Traversal and Application Denial of Service, depending on the context. The vulnerability occurs when user-supplied input is not properly sanitized before being passed to the unserialize PHP function.
In the case of PHP code injection attacks, an attacker takes advantage of a script that contains system functions/calls to read or execute malicious code on a remote server. This is synonymous to having a backdoor shell and under certain circumstances can also enable privilege escalation.