Description:
------------
in php 7 debug_backtrace no longer shows argument values as they were passed to the function, showing instead the values at the moment of the next call in the trace

official php page (http://php.net/manual/en/migration70.incompatible.php) doesn't mention this change, only informing of similar change to behavior of func_get_arg(s), but in comments to bug 70547 https://bugs.php.net/bug.php?id=70547 change was described as intended performance optimization with reference to these notes: http://git.php.net/?p=php-src.git;a=blob;f=UPGRADING;h=aa5f82c49abf4e36476b328c2fc8820d4672c72a;hb=refs/heads/PHP-7.0#l190

While change to the func_get_arg(s) seem harmless (they can be simply used in the very beginning of the function), backtace that shows function calls with arguments values different from what were passed is very confusing. There're many situations when function gets an argument, modifies it, passes to yet another function etc. When debugging them, new backtrace confuses instead of helping.

Suggestion: if it is important for the performance not to store the copies of the arguments, add ini setting that will toggle between old and new behavior, giving developer an option to choose between better debugging capabilities and better performance

Test script:
---------------
function func($arg)
{
$arg['something'] = 2;
print_r(debug_backtrace()[0]);
}
func(['something' => 1]);

Expected result:
----------------
Array
(
    [function] => func
    [args] => Array
        (
            [0] => Array
                (
                    [something] => 1
                )

        )

)

Actual result:
--------------
Array
(
    [function] => func
    [args] => Array
        (
            [0] => Array
                (
                    [something] => 2
                )

        )

)