Can't decode php-session-format serialization outside a session
The only way to decode php-session-format serialization (different from normal php serialize) is by calling php session_decode, which can only be called if there is an active session. If there is no active session, you can't decode session data without starting one first.
Significance: Missing Features
Language features make developers' lives easier. Often, language features are not complex for the language designers to implement (barring unnecessary complications in the internals), but can save developers hours of time. Missing language features are disrespectful to developers and encourage dirty hacks, "clever" solutions, and kludgy workarounds to achieve the desired functionality.