PHP Internals — A Low-Probability Notice in Session GC中文EN

If you installed PHP via apt on ubuntu/Debian, then when using Sessions you may occasionally run into this notice:

PHP Notice: session_start(): ps_files_cleanup_dir:
   opendir(/var/lib/php5) failed: Permission denied (13)
   in /home/laruence/www/htdocs/index.php on line 22

This is because in PHP, when using file_handler as the Session save handler, there's a probability that the Session GC process runs on each session_start:

//abridged
        int nrdels = -1;
        nrand = (int) ((float) PS(gc_divisor) * php_combined_lcg(TSRMLS_C));
        if (nrand < PS(gc_probability)) {
            PS(mod)->s_gc(&PS(mod_data), PS(gc_maxlifetime), &nrdels TSRMLS_CC);
        }
//abridged

The reason for this warning is that in the apt PHP, the default session directory /var/lib/php5 has permissions of 733 with the sticky bit, i.e.

drwx-wx-wt  root  root

And PHP workers generally run as a non-root user, so they have no permission to open this directory (though because they can write, normal Session file access isn't affected). So the following code in s_gc triggers the Notice described above:

//for the file handler, s_gc indirectly calls ps_files_cleanup_dir:
   dir = opendir(dirname);
    if (!dir) {
        php_error_docref(NULL TSRMLS_CC, E_NOTICE,
           "ps_files_cleanup_dir: opendir(%s) failed: %s (%d)",
           dirname, strerror(errno), errno);
        return (0);
    }

Of course, on ubuntu/Debian there's still GC cleanup — it's just done by an external cron process. By default it's in /etc/cron.d/php5:

09,39 *     * * *     root   [ -x /usr/lib/php5/maxlifetime ]
&& [ -d /var/lib/php5 ] && find /var/lib/php5/
 -type f -cmin +$(/usr/lib/php5/maxlifetime) -print0
| xargs -n 200 -r -0 rm

Also, you can see that when deciding whether s_gc runs, there are two key variables: PS(gc_divisor) and PS(gc_probability). These correspond to two same-named session runtime settings:
session.gc_probability and session.gc_divisor, defaulting to 1 and 100 respectively.
And php_combined_lcg is a random number generator producing a value in the 0–1 range, so the check above is equivalent to:

 rand < probability / gc_divisor

That is, by default the GC process is triggered roughly once every 100 times. That's why you only see this Notice with a small probability.
To silence this Notice, simply set:
session.gc_probability = 0, so s_gc has no chance to run at all.
Of course, you can also change the permissions on that folder...
Finally, thanks to CFC4N for reporting this issue.