Here's an example which shows how zany things are:

<?php

$list = array('6e2', '1', 2, '3', 'z', 'a');
for ($ii = 0; $ii < 10000; $ii++) {
  shuffle($list);
  sort($list);
  echo implode(', ', $list)."\n";
}

This repeatedly shuffles a list, then sorts it. One might reasonably expect that the output would be identical, but it isn't. We get three different sorted orders, depending on the input order.

$ php -f test.php | sort | uniq -c
4118 1, 2, 3, 6e2, a, z
3949 1, 3, 6e2, a, z, 2
1933 1, a, z, 2, 3, 6e2

I think this is because < on mixed types is not transitive, so the actual comparisons performed depend on the input, and lead to multiple sort orders.

In some places, we prefix strings with "~" to prevent this (this forces string casting). I started doing this a while ago, because I didn't realize SORT_STRING exists, and didn't fully realize the implications of PHP's goofy behavior.

We should likely:

• Identify callsites to sort(), ksort(), asort(), arsort(), rsort() and krsort() and add SORT_STRING after verifying they are safe.
• Change msort() to use SORT_STRING.
• Remove superfluous "~" and similar characters from sort keys, eventually.
• Add a lint warning about this nonsense.

(It's technically deterministic, I suppose, just very unpredictable.)

How about adding the $sort_flag to msort as a parameter? It could very well be useful to explicitly sort things numerically?

I suspect that's never useful in this codebase, but could be wrong.