# session_gc

(PHP 7 >= 7.1.0)

session_gc — Perform session data garbage collection

### 说明

session_gc ( void ) : int

session_gc() is used to perform session data GC(garbage collection). PHP does probability based session GC by default.

Probability based GC works somewhat but it has few problems. 1) Low traffic sites' session data may not be deleted within the preferred duration. 2) High traffic sites' GC may be too frequent GC. 3) GC is performed on the user's request and the user will experience a GC delay.

Therefore, it is recommended to execute GC periodically for production systems using, e.g., "cron" for UNIX-like systems. Make sure to disable probability based GC by setting session.gc_probability to 0.

### 返回值

session_gc() returns number of deleted session data for success, `FALSE` for failure.

Old save handlers do not return number of deleted session data, but only success/failure flag. If this is the case, number of deleted session data became 1 regardless of actually deleted data.

### 范例

Example #1 session_gc() example for task managers like cron

    <?php
    // Note: This script should be executed by the same user of web server process.

    // Need active session to initialize session data storage access.
    session_start();

    // Executes GC immediately
    session_gc();

    // Clean up session ID created by session_gc()
    session_destroy();
    ?>

Example #2 session_gc() example for user accessible script

    <?php
    // Note: session_gc() is recommended to be used by task manager script, but
    // it may be used as follows.

    // Used for last GC time check
    $gc_time = '/tmp/php_session_last_gc';
    $gc_period = 1800;

    session_start();
    // Execute GC only when GC period elapsed.
    // i.e. Calling session_gc() every request is waste of resources.
    if (file_exists($gc_time)) {
        if (filemtime($gc_time) < time() - $gc_period) {
            session_gc();
            touch($gc_time);
        }
    } else {
        touch($gc_time);
    }
    ?>

### 参见

- session_start() - 启动新会话或者重用现有会话
- session_destroy() - 销毁一个会话中的全部数据
- session.gc_probability