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<h2 class="title">Custom Session Handlers</h2>
<p class="para">
To implement database storage, or any other storage method, you
will need to use <span class="function"><a href="function.session-set-save-handler.html" class="function">session_set_save_handler()</a></span> to
create a set of user-level storage functions. As of PHP 5.4.0 you may create session handlers
using the <a href="class.sessionhandlerinterface.html" class="classname">SessionHandlerInterface</a> or extend internal PHP handlers by inheriting
from <a href="class.sessionhandler.html" class="classname">SessionHandler</a>.
</p>
<p class="para">
The callbacks specified in <span class="function"><a href="function.session-set-save-handler.html" class="function">session_set_save_handler()</a></span> are methods
called by PHP during the life-cycle of a session: <em><code class="parameter">open</code></em>, <em><code class="parameter">read</code></em>,
<em><code class="parameter">write</code></em> and <em><code class="parameter">close</code></em> and for the housekeeping tasks:
<em><code class="parameter">destroy</code></em> for deleting a session and <em><code class="parameter">gc</code></em> for periodic garbage
collection.
</p>
<p class="para">
Therefore, PHP always requires session save handlers. The default is usually the
internal 'files' save handler. A custom save handler can be set using
<span class="function"><a href="function.session-set-save-handler.html" class="function">session_set_save_handler()</a></span>. Alternative internal save handlers are also be
provided by PHP extensions, such as <em><code class="parameter">sqlite</code></em>,
<em><code class="parameter">memcache</code></em> and <em><code class="parameter">memcached</code></em> and can be set with
<a href="session.configuration.html#ini.session.save-handler" class="link">session.save_handler</a>.
</p>
<p class="para">
When the session starts, PHP will internally call the <em><code class="parameter">open</code></em> handler followed by the
<em><code class="parameter">read</code></em> callback which should return an encoded string exactly as it was originally
passed for storage. Once the <em><code class="parameter">read</code></em> callback returns the encoded string, PHP will
decode it and then populate the resulting array into the <var class="varname"><var class="varname"><a href="reserved.variables.session.html" class="classname">$_SESSION</a></var></var> superglobal.
</p>
<p class="para">
When PHP shuts down (or when <span class="function"><a href="function.session-write-close.html" class="function">session_write_close()</a></span> is called),
PHP will internally encode the <var class="varname"><var class="varname"><a href="reserved.variables.session.html" class="classname">$_SESSION</a></var></var> superglobal and pass this
along with the session ID to the the <em><code class="parameter">write</code></em> callback.
After the <em><code class="parameter">write</code></em> callback has finished, PHP will internally invoke the
<em><code class="parameter">close</code></em> callback handler.
</p>
<p class="para">
When a session is specifically destroyed, PHP will call the <em><code class="parameter">destroy</code></em> handler with
the session ID.
</p>
<p class="para">
PHP will call the <em><code class="parameter">gc</code></em> callback from time to time to
expire any session records according to the set max lifetime of a session.
This routine should delete all records from persistent storage which were
last accessed longer than the <em><code class="parameter">$lifetime</code></em>.
</p>
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