In PHP, what do you expect the following to output?

<?php
$x = "12a";
$x++;
echo $x;

If you guessed “error” or 12a, then you’re still sane, but sadly wrong.

The answer is 12b, due to PHP’s hybrid base-26/base-10 numeric system in strings.

Let’s try another one:

<?php
$x = "12z";
$x++;
echo $x;

Any guesses?

The answer is 13a. z wraps over to a (base 26), and then carries over to change 2 to 3. Of course!

Last one:

<?php
$x = "0d9";
$x++;
$x++;

echo $x;
echo gettype($x);

The answer is 1, and the type is double.

The first increment operator wraps 0d9 into 0e0 by the rules above. This happens to be the scientific notation for 0, unfortunately.

When we increment a string that looks like a number in scientific notation, PHP instead juggles it to be a number, then performs the operation on that!

This isn’t likely to come up in any real code, but that doesn’t stop this from being one of those bizarre features in PHP.