src/Mutator/Arithmetic/Addition.php
public static function mutates(array &$tokens, $index)
    {
        $t = $tokens[$index];
        if (!is_array($t) && $t == '+') {
            $tokenCount = count($tokens);
            for ($i = $index + 1; $i < $tokenCount; $i++) {
                // check for short array syntax
                if (!is_array($tokens[$i]) && $tokens[$i][0] == '[') {
                    return false;
                }

                // check for long array syntax
                if (is_array($tokens[$i]) && $tokens[$i][0] == T_ARRAY && $tokens[$i][1] == 'array') {
                    return false;
                }

                // if we're at the end of the array
                // and we didn't see any array, we
                // can probably mutate this addition
                if (!is_array($tokens[$i]) && $tokens[$i] == ';') {
                    return true;
                }
            }
            return true;
        }

        return false;
    }
public static function mutates(array &$tokens, $index)
    {
        $t = $tokens[$index];
        if (!is_array($t) && $t == '+') {
            $tokenCount = count($tokens);
            for ($i = $index + 1; $i < $tokenCount; $i++) {
                // check for short array syntax
                if (!is_array($tokens[$i]) && $tokens[$i][0] == '[') {
                    return false;
                }

                // check for long array syntax
                if (is_array($tokens[$i]) && $tokens[$i][0] == T_ARRAY && $tokens[$i][1] == 'array') {
                    return false;
                }

                // if we're at the end of the array
                // and we didn't see any array, we
                // can probably mutate this addition
                if (!is_array($tokens[$i]) && $tokens[$i] == ';') {
                    return true;
                }
            }
            return true;
        }

        return false;
    }
Not all additions can be mutated.

The PHP language allows union of arrays : $var = ['foo' => true] + ['bar' => true]
see http://php.net/manual/en/language.operators.array.php for details.

So for this case, we can't create a mutation.

@param array $tokens
@param $index
@return bool