Why double underscore (__) in php?
<p>Here are many more function with double underscore.</p>

<pre><code>__construct, 
__destruct, 
__call, 
__callStatic, 
__get, 
__set, 
__isset, 
__unset, 
__sleep, 
__wakeup, 
__toString, 
__invoke, 
__set_state 
__clone
</code></pre>

<p>I only need why this underscore is used before these functions.</p>
