How Long Do PHP Sessions Really Last?

Session Duration in PHP

When working with PHP, it's essential to consider the duration of active user sessions. The session configuration determines how long a session will remain active before it expires.

Understanding Session Configuration:

To determine the session duration, refer to the following configuration settings:

• session.gc_maxlifetime: Specifies the maximum lifetime in seconds of the session data from the last modification.

PHP Session Garbage Collection:

However, PHP introduces an additional layer of complexity through its session garbage collection mechanism. This mechanism doesn't immediately remove expired sessions after the specified session.gc_maxlifetime.

The session garbage collector is triggered when session_start is called, but only with a certain probability. This probability is defined by the settings:

• session.gc_probability: Probability of initiating garbage collection (default: 1)
• session.gc_divisor: Divisor to determine the probability (default: 100)

As a result, even if a session is technically expired (based on session.gc_maxlifetime), it may persist for a longer duration due to the probability-based garbage collection.

Recommendation:

To ensure reliable session expiry, it's recommended to implement your own session timeout mechanism. This approach provides more control over session lifetimes and prevents potential security risks associated with extended session durations.

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