More test code
https://3v4l.org/TZaVe (array)
https://3v4l.org/WMMQa (string)
https://3v4l.org/OEOA1 (return value check)

Removing error from call_user_func('foo', $value), when $value is object, is preferred. foo() could be.

function foo(&$value) {
  if (is_string($value)) {
      $value = 'update $value';
  } else if (is_object($value)) {
      $value->some_var = 'add some_var';
  } else {
      trigger_error('You should pass string or object');
  }
}

These changes are intentional -- they bring the call_user_func VM implementation in line with the actual function. call_user_func() categorically cannot pass references, so you get a warning. (At least this is part of it.)

However, in PHP 7.1 we have changed call_user_func() to still proceed with the call even if such a warning occurs. Previously the call would be aborted (which would be a BC break). Now it will continue as normal, only with the warning.

@nikic
I agree that errors should be raised for bogus calls like

call_user_func('foo', 'abc'); // Literal cannot be reference
call_user_func('foo', [$o]); // There is no modifiable value
call_user_func('foo', array_slice($a, 0, 1)); // There is no modifiable value

when foo() is

function foo(&$value) { ... }

These call_user_func() calls cannot be pass by reference. I'll update http://php.net/manual/en/function.call-user-func.php and others to document behavior precisely.

The issue remained is error on

call_user_func('foo', $o);

$o could be actual value like $o="abc". (We also should consider functions return reference which should be able to be updated)

Could this error be fixed/removed? If not, I'll just document call_user_func() parameter cannot be a reference for any type of variables.

> The issue remained is error on
> 
> call_user_func('foo', $o);

Just don't use call_user_func; just call the callable directly
instead[1], what is possible for ages.  It may be useful to
document that on the call_user_func() page.

The limitation regarding pass by reference is already documented.

[1] <https://3v4l.org/NHHQR>