From manual page: https://php.net/function.preg-match-all

It says:

matches
Array of all matches in multi-dimensional array ordered according to flags.

Among the flags, there are:

PREG_PATTERN_ORDER
Orders results so that $matches[0] is an array of full pattern matches, $matches[1] is an array of strings matched by the first parenthesized subpattern, and so on.

and

PREG_SET_ORDER
Orders results so that $matches[0] is an array of first set of matches, $matches[1] is an array of second set of matches, and so on.

which are mutually exclusive (and indeed it says "note that it doesn't make sense to use PREG_PATTERN_ORDER together with PREG_SET_ORDER")

So which one is it by default?

Given that the documented default value of $flags is

int $flags = 0,

one would deduce that is neither of the above, and therefore what is it??

The reality is that the default is the behavior described in PREG_PATTERN_ORDER. So, either (A) the default value of the $flags parameter is actually PREG_PATTERN_ORDER and not 0 as stated, or (B) the default behavior is the same as the one explicitly enforced by the flag PREG_PATTERN_ORDER and this is not said anywhere. The only way to know is to try.