一、php session函数的原理
session_id作用
重用现有的回话，如果没有就创建，感觉这个函数作用不是很大，我在php这边打印不好使。
session_start作用
1、没有会话，根据session.name = PHPSESSID生成Cookie，并发送给客户端。
2、打开与介质的连接通道，读取相应的id内容到变量$_SESSION中。
session_unset作用
清空变量$_SESSION
session_destroy作用
删除介质中的数据并关闭与介质的连接，但是本地$_SESSION如果不调用session_unset还存在
二、session的垃圾回收机制
session.gc_maxlifetime
session.gc_probability
session.gc_divisor
session.gc_divisor 与 session.gc_probability 合起来定义了在每个会话初始化时启动 gc（garbage collection 垃圾回收）进程的概率。此概率用 gc_probability/gc_divisor 计算得来。例如 1/100 意味着在每个请求中有 1% 的概率启动 gc 进程。session.gc_divisor 默认为 100。
比如：session.gc_maxlifetime=30，session.gc_divisor=1000，session.gc_probability=1，就表示每一千个用户调用session_start()的时候，就百分百的会执行一次垃圾回收机制，将磁盘上没用的session文件删除。
注意：一般对于一些大型的门户网站，建议将session.gc_divisor调大一点，减少开销
接下来，我通过一个例子演示下，如何配置才能让调用gc（垃圾回收）进程呢！
通过配置php.ini文件，修改以下几个信息：
- session.gc_maxlifetime = 60//当session文件在60s后还没有被访问的话，则该session文件将会被视为“垃圾文件”，并且等待gc（垃圾回收）进程的调用的时候被清理掉
- session.gc_probability = 1000
因为gc进程被调用的概率是通过gc_probability/gc_divisor 计算得来的，这里我将session.gc_probability改成1000，而session.gc_divisor 默认情况下也是1000。则gc进程在每次执行session_start()函数的时候都会被调用到。
以下我通过截图简单的说明下：
我开启三个会话，则创建三个对应的session文件，当每个文件在30秒内都没被调用的话，就会被当成是“垃圾文件”，等到gc进程调用的时候，“垃圾文件”就会被unlink，因为之前我已经通过修改php.ini配置文件，将gc被调用的概率改成百分百，所以接下来，如果我重新使用任何一个浏览器刷新下页面的时候，三个session文件，应该只剩下一个了
三、php相关session配置文件
[Session]
; Handler used to store/retrieve data.
; http://php.net/session.save-handler
session.save_handler = files
; Argument passed to save_handler. In the case of files, this is the path
; where data files are stored. Note: Windows users have to change this
; variable in order to use PHP's session functions.
;
; The path can be defined as:
;
session.save_path = "N;/path"
;
; where N is an integer. Instead of storing all the session files in
; /path, what this will do is use subdirectories N-levels deep, and
; store the session data in those directories. This is useful if
; your OS has problems with many files in one directory, and is
; a more efficient layout for servers that handle many sessions.
;
; NOTE 1: PHP will not create this directory structure automatically.
; You can use the script in the ext/session dir for that purpose.
; NOTE 2: See the section on garbage collection below if you choose to
; use subdirectories for session storage
;
; The file storage module creates files using mode 600 by default.
; You can change that by using
;
; session.save_path = "N;MODE;/path"
;
; where MODE is the octal representation of the mode. Note that this
; does not overwrite the process's umask.
; http://php.net/session.save-path
;session.save_path = "/tmp"
; Whether to use strict session mode.
; Strict session mode does not accept uninitialized session ID and regenerate
; session ID if browser sends uninitialized session ID. Strict mode protects
; applications from session fixation via session adoption vulnerability. It is
; disabled by default for maximum compatibility, but enabling it is encouraged.
; https://wiki.php.net/rfc/strict_sessions
session.use_strict_mode = 0
; Whether to use cookies.
; http://php.net/session.use-cookies
session.use_cookies = 1
; http://php.net/session.cookie-secure
session.cookie_secure = 1
; This option forces PHP to fetch and use a cookie for storing and maintaining
; the session id. We encourage this operation as it's very helpful in combating
; session hijacking when not specifying and managing your own session id. It is
; not the be-all and end-all of session hijacking defense, but it's a good start.
; http://php.net/session.use-only-cookies
session.use_only_cookies = 1
; Name of the session (used as cookie name).
; http://php.net/session.name
session.name = PHPSESSID
; Initialize session on request startup.
; http://php.net/session.auto-start
session.auto_start = 0
; Lifetime in seconds of cookie or, if 0, until browser is restarted.
; http://php.net/session.cookie-lifetime
session.cookie_lifetime = 0
; The path for which the cookie is valid.
; http://php.net/session.cookie-path
session.cookie_path = /
; The domain for which the cookie is valid.
; http://php.net/session.cookie-domain
session.cookie_domain = aliyun.com
; Whether or not to add the httpOnly flag to the cookie, which makes it inaccessible to browser scripting languages such as JavaScript.
; http://php.net/session.cookie-httponly
session.cookie_httponly =
; Handler used to serialize data. php is the standard serializer of PHP.
; http://php.net/session.serialize-handler
session.serialize_handler = igbinary
; Defines the probability that the 'garbage collection' process is started
; on every session initialization. The probability is calculated by using
; gc_probability/gc_divisor. Where session.gc_probability is the numerator
; and gc_divisor is the denominator in the equation. Setting this value to 1
; when the session.gc_divisor value is 100 will give you approximately a 1% chance
; the gc will run on any give request.
; Default Value: 1
; Development Value: 1
; Production Value: 1
; http://php.net/session.gc-probability
session.gc_probability = 1
; Defines the probability that the 'garbage collection' process is started on every
; session initialization. The probability is calculated by using the following equation:
; gc_probability/gc_divisor. Where session.gc_probability is the numerator and
; session.gc_divisor is the denominator in the equation. Setting this value to 1
; when the session.gc_divisor value is 100 will give you approximately a 1% chance
; the gc will run on any give request. Increasing this value to 1000 will give you
; a 0.1% chance the gc will run on any give request. For high volume production servers,
; this is a more efficient approach.
; Default Value: 100
; Development Value: 1000
; Production Value: 1000
; http://php.net/session.gc-divisor
session.gc_divisor = 1000
; After this number of seconds, stored data will be seen as 'garbage' and
; cleaned up by the garbage collection process.
; http://php.net/session.gc-maxlifetime
session.gc_maxlifetime = 1440
; NOTE: If you are using the subdirectory option for storing session files
; (see session.save_path above), then garbage collection does *not*
; happen automatically. You will need to do your own garbage
; collection through a shell script, cron entry, or some other method.
; For example, the following script would is the equivalent of
; setting session.gc_maxlifetime to 1440 (1440 seconds = 24 minutes):
; find /path/to/sessions -cmin +24 -type f | xargs rm
; PHP 4.2 and less have an undocumented feature/bug that allows you to
; to initialize a session variable in the global scope, even when register_globals
; is disabled. PHP 4.3 and later will warn you, if this feature is used.
; You can disable the feature and the warning separately. At this time,
; the warning is only displayed, if bug_compat_42 is enabled. This feature
; introduces some serious security problems if not handled correctly. It's
; recommended that you do not use this feature on production servers. But you
; should enable this on development servers and enable the warning as well. If you
; do not enable the feature on development servers, you won't be warned when it's
; used and debugging errors caused by this can be difficult to track down.
; Default Value: On
; Development Value: On
; Production Value: Off
; http://php.net/session.bug-compat-42
session.bug_compat_42 = Off
; This setting controls whether or not you are warned by PHP when initializing a
; session value into the global space. session.bug_compat_42 must be enabled before
; these warnings can be issued by PHP. See the directive above for more information.
; Default Value: On
; Development Value: On
; Production Value: Off
; http://php.net/session.bug-compat-warn
session.bug_compat_warn = On
; Check HTTP Referer to invalidate externally stored URLs containing ids.
; HTTP_REFERER has to contain this substring for the session to be
; considered as valid.
; http://php.net/session.referer-check
session.referer_check =
; How many bytes to read from the file.
; http://php.net/session.entropy-length
;session.entropy_length = 0
; Specified here to create the session id.
; http://php.net/session.entropy-file
; Defaults to /dev/urandom
; On systems that don't have /dev/urandom but do have /dev/arandom, this will default to /dev/arandom
; If neither are found at compile time, the default is no entropy file.
; On windows, setting the entropy_length setting will activate the
; Windows random source (using the CryptoAPI)
;session.entropy_file = /dev/urandom
; Set to {nocache,private,public,} to determine HTTP caching aspects
; or leave this empty to avoid sending anti-caching headers.
; http://php.net/session.cache-limiter
session.cache_limiter = nocache
; Document expires after n minutes.
; http://php.net/session.cache-expire
session.cache_expire = 180
; trans sid support is disabled by default.
; Use of trans sid may risk your users' security.
; Use this option with caution.
; - User may send URL contains active session ID
; to other person via. email/irc/etc.
; - URL that contains active session ID may be stored
; in publicly accessible computer.
; - User may access your site with the same session ID
; always using URL stored in browser's history or bookmarks.
; http://php.net/session.use-trans-sid
session.use_trans_sid = 0
; Select a hash function for use in generating session ids.
; Possible Values
; 0 (MD5 128 bits)
; 1 (SHA-1 160 bits)
; This option may also be set to the name of any hash function supported by
; the hash extension. A list of available hashes is returned by the hash_algos()
; function.
; http://php.net/session.hash-function
session.hash_function = 0
; Define how many bits are stored in each character when converting
; the binary hash data to something readable.
; Possible values:
; 4 (4 bits: 0-9, a-f)
; 5 (5 bits: 0-9, a-v)
; 6 (6 bits: 0-9, a-z, A-Z, "-", ",")
; Default Value: 4
; Development Value: 5
; Production Value: 5
; http://php.net/session.hash-bits-per-character
session.hash_bits_per_character = 5
; The URL rewriter will look for URLs in a defined set of HTML tags.
; form/fieldset are special; if you include them here, the rewriter will
; add a hidden <input> field with the info which is otherwise appended
; to URLs. If you want XHTML conformity, remove the form entry.
; Note that all valid entries require a "=", even if no value follows.
; Default Value: "a=href,area=href,frame=src,form=,fieldset="
; Development Value: "a=href,area=href,frame=src,input=src,form=fakeentry"
; Production Value: "a=href,area=href,frame=src,input=src,form=fakeentry"
; http://php.net/url-rewriter.tags
url_rewriter.tags = "a=href,area=href,frame=src,input=src,form=fakeentry"
; Enable upload progress tracking in $_SESSION
; Default Value: On
; Development Value: On
; Production Value: On
; http://php.net/session.upload-progress.enabled
;session.upload_progress.enabled = On
; Cleanup the progress information as soon as all POST data has been read
; (i.e. upload completed).
; Default Value: On
; Development Value: On
; Production Value: On
; http://php.net/session.upload-progress.cleanup
;session.upload_progress.cleanup = On
; A prefix used for the upload progress key in $_SESSION
; Default Value: "upload_progress_"
; Development Value: "upload_progress_"
; Production Value: "upload_progress_"
; http://php.net/session.upload-progress.prefix
;session.upload_progress.prefix = "upload_progress_"
; The index name (concatenated with the prefix) in $_SESSION
; containing the upload progress information
; Default Value: "PHP_SESSION_UPLOAD_PROGRESS"
; Development Value: "PHP_SESSION_UPLOAD_PROGRESS"
; Production Value: "PHP_SESSION_UPLOAD_PROGRESS"
; http://php.net/session.upload-progress.name
;session.upload_progress.name = "PHP_SESSION_UPLOAD_PROGRESS"
; How frequently the upload progress should be updated.
; Given either in percentages (per-file), or in bytes
; Default Value: "1%"
; Development Value: "1%"
; Production Value: "1%"
; http://php.net/session.upload-progress.freq
;session.upload_progress.freq = "1%"
; The minimum delay between updates, in seconds
; Default Value: 1
; Development Value: 1
; Production Value: 1
; http://php.net/session.upload-progress.min-freq
;session.upload_progress.min_freq = "1"
; Only write session data when session data is changed. Enabled by default.
; http://php.net/session.lazy-write
;session.lazy_write = On参考：https://www.cnblogs.com/hongfei/archive/2012/06/17/2552434.html
