Get and/or set the current session id

# session_id

(PHP 4, PHP 5, PHP 7, PHP 8)

session_id — Get and/or set the current session id

### Description

session_id(?string `$id` = `null`): string|false

session_id() is used to get or set the session id for the current session.

The constant `SID` can also be used to retrieve the current name and session id as a string suitable for adding to URLs. See also Session handling.

### Parameters

`id`

  If `id` is specified and not `null`, it will replace the current session id. session_id() needs to be called before session_start() for that purpose. Depending on the session handler, not all characters are allowed within the session id. For example, the file session handler only allows characters in the range `[a-zA-Z0-9,-]`!

  > Note: When using session cookies, specifying an `id` for session_id() will always send a new cookie when session_start() is called, regardless if the current session id is identical to the one being set.

### Return Values

session_id() returns the session id for the current session or the empty string (`""`) if there is no current session (no current session id exists). On failure, `false` is returned.

### Changelog

Version | Description
--- | ---
8.0.0 | `id` is nullable now.

### See Also

- session_regenerate_id() - Update the current session id with a newly generated one
- session_start() - Start new or resume existing session
- session_set_save_handler() - Sets user-level session storage functions
- session.save_handler

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