Copy On Write and Assign By Reference perform different on PHP5 and PHP7

Hi, internals, I have a question about Assign By Reference and I posted on StackOverflow, I'd like to know the reason behind it, and I did not get any this kind of answer, can anyone give me some clues. --- We have a piece of simple code: 1 <?php 2 $i = 2; 3 $j = &$i; 4 echo (++$i) + (++$i); On PHP5, it outputs 8, because: $i is a reference, when we increase $i by ++i, it will change the zval rather than make a copy, so line 4 will be 4 + 4 = 8. This is Assign By Reference. If we comment line 3, it will output 7, every time we change the value by increasing it, PHP will make a copy, line 4 will be 3 + 4 = 7. This is Copy On Write. But in PHP7, it always outputs 7. I've checked the changes in PHP7: http://php.net/manual/en/migration70.incompatible.php, but I did not get any clue. Here is the result of the code on PHP5 / PHP7: https://3v4l.org/USTHR And there is the link on Stackoverflow: http://stackoverflow.com/questions/36338661/php-copy-on-write-and-assign-by-reference-perform-different-on-php5-and-php7 Thanks guys. Best, -- Huqiu Liao http://www.liaohuqiu.net