unserialize() is a PHP function that takes serialized data and rebuilds the original value, such as arrays or objects. It is powerful because it can restore application state quickly, but it is dangerous when the input comes from a user, a request parameter, a cookie, or any other untrusted source. If attacker-controlled data is passed into unserialize(), PHP may instantiate objects and invoke magic methods during reconstruction.
That behavior matters in cyber security because it can create PHP object injection vulnerabilities. An attacker may abuse existing classes as a gadget chain to trigger file writes, command execution, or other unintended actions. In real defenses, developers avoid unserialize() on untrusted data, prefer safer formats like JSON, and restrict allowed classes when deserialization is unavoidable. Security reviews should also trace every path that reaches unserialize(), because a single unsafe call can turn normal input handling into a route toward remote code execution.