Description:
------------
When modifying an argument array that includes an entry for &$this, the assignment operation seems to de-reference the &$this argument. As shown in the test script, line 6 and line 8 are identical, and line 7 assigns the same value to index [0] of the array.

The issue is specific to PHP 7.1 (I have tested with 7.1.8 and 7.1.9), but it does not appear in PHP 7.0.

var_dump before and after the assignment operation has the same printout. There is a difference in var_dump output between PHP 7.0 and 7.1. In 7.1, the var_dump output is missing the "&" for array entry [1], while the "&" symbol is present in PHP 7.0.

This issue causes problems for WordPress sites, as it uses similar logic when running filters (WP_Hook::apply_filters).

Test script:
---------------
<?php
function test_call($first, &$a) {
  echo '<div>here</div>';
}
function test_warning($args) {
  call_user_func_array( 'test_call', $args ); // this line works
  $args[ 0 ] = 1;
  call_user_func_array( 'test_call', $args ); // this line causes error
}
class TestWarningClass{
  function __construct() {
      $args = array( 1, &$this );
      test_warning($args);
  }
}
$test = new TestWarningClass;

Expected result:
----------------
No warning. Two lines of "here" are printed.

Actual result:
--------------
Warning: Parameter 2 to test_call() expected to be a reference, value given