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Rule ``backtick_to_shell_exec``
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Converts backtick operators to ``shell_exec`` calls.

Description
-----------

Conversion is done only when it is non risky, so when special chars like
single-quotes, double-quotes and backticks are not used inside the command.

Examples
--------

Example #1
~~~~~~~~~~

.. code-block:: diff

   --- Original
   +++ New
   @@ -1,3 +1,3 @@
    <?php
   -$plain = `ls -lah`;
   -$withVar = `ls -lah $var1 ${var2} {$var3} {$var4[0]} {$var5->call()}`;
   +$plain = shell_exec("ls -lah");
   +$withVar = shell_exec("ls -lah $var1 ${var2} {$var3} {$var4[0]} {$var5->call()}");