Today, Anthony Ferrara mentioned some strange behaviors about the differences between HHVM and PHP runtimes. If you haven't heard of HHVM, definitely go check it out. It is a PHP execution engine built by Facebook that is extremely fast, which is why Facebook built and is using it.

However, as Anthony mentioned in his twitter post, HHVM isn't necessarily a direct drop-in replacement for PHP. And one of his points is about the two engines' behaviors concerning PHP_INT_MAX.

Making Max Overflow

So, Anthony wasn't highlighting the difference of PHP_INT_MAX, but rather what happens when you add to that. Conventional static-typed languages will have the integer overflow, since it has to remain an integer. However, PHP is a dynamically-typed language, and so it adjusts itself to become a floating-point value so that it will accept the addition accurately.

But HHVM doesn't seem to follow the same rules. Let's prove it.

The Experiment

We can show the values and types of PHP_INT_MAX and PHP_INT_MAX + 1 in a short script where we var_dump each value.

<?php

echo 'PHP_INT_MAX: ';
echo var_dump(PHP_INT_MAX) . PHP_EOL;

echo 'PHP_INT_MAX + 1: ';
echo var_dump(PHP_INT_MAX + 1) . PHP_EOL;

The Results

Below, we run the script using standard PHP version 5.5 (64-bit), and we see our expected output.

# PHP Output
$ php test.php
  PHP_INT_MAX: int(9223372036854775807)
  PHP_INT_MAX + 1: float(9.2233720368548E+18)

However, running the script with HHVM, we get a different result.

# HHVM Output
$ hhvm test.php
  PHP_INT_MAX: int(9223372036854775807)
  PHP_INT_MAX + 1: int(-9223372036854775808)

We can see that HHVM takes the static-typed approach to integers and overflows them to a negative value.

Lessons Learned

This helps highlight one of the reasons why HHVM isn't yet (or may never be) a drop-in replacement for PHP. If you wish to use HHVM, make sure your code is tested to be confident that your application will work properly.

Whether or not PHP/HHVM should behave the way it does is a question to be answered by minds greater than my own. But for now, I hope this may save you some headache down the road.

Again, thanks to Anthony Ferrara for giving me a lesson in PHP and computer science today.