Unserialize

Unserialize is a function in PHP used to convert a string produced by serialize back into a PHP value. It is the inverse operation of serialize and is commonly used to store or transfer complex data structures.

When data is unserialized, PHP reconstructs the original types, including arrays and objects. Scalars, arrays, and nested structures are restored as they were before serialization. For objects, PHP may instantiate the corresponding class in the current context and then populate its properties. The process can trigger object lifecycle methods such as __wakeup, which runs during unserialization, and, in some cases, other magic methods during object destruction or string conversion.

Security considerations are central to using unserialize. Deserializing data from untrusted sources can lead to object injection vulnerabilities, where malicious input exploits class behavior to execute code, modify data, or perform other unintended actions. Best practices include avoiding unserialize on untrusted data, validating sources, using formats such as JSON when possible, and restricting which classes may be instantiated. Since PHP provides an option called allowed_classes for unserialize, developers can limit or disable object instantiation during deserialization by configuring this option.

Alternatives and related concepts include using json_encode and json_decode for data interchange, which avoids PHP object instantiation issues, and adopting modern serialization methods such as __serialize and __unserialize in newer PHP versions, or implementing the Serializable interface for explicit control over serialization behavior.

In summary, unserialize reconstitutes PHP values from a serialized string, but it carries security risks if used with untrusted input, making careful handling and appropriate restrictions essential.