Not known and most likely user error. Please provide a reproducing script.

it was really an user error, sorry.
i found than "++" operator doesn't make type conversion from boolean to int, and if variable is boolean and equals TRUE than after ++ it remains as TRUE, so

$a = TRUE; echo ($a++).$a;

produces output "11"
this behavior of increment operator was unexpected to me and i suppose that it can be the same to others, so i wrote an user note at "type juggling" part at online documentation

Reclassified.

Please explain how this is to be documented (why this isn't a bug) as I fail to understand the seemingly inconsistent behavior of this example:

$t = true;
$a = $t + 1; (same as $t += 1; // 2)
$b = ++$t;

print "a: $a b: $b"; // a: 2 b: 1

In otherwords, incrementing/decrementing a boolean keeps it as boolean while adding an integer, such as 1, changes it to an integer.

Reclassifying as a scripting engine problem so php-dev gets this report.  Please provide a documentable reason for this behavior.

it's still not a bug, and there was some discussion about this on php-dev or some other list a while ago..can't find that now thouhg, but there was also "fix" for this, but it was reverted. (iirc)

compare this to

$a = "a";
$a = $a + 1;

and 

$a = "a";
$a = $a++;

++ is defined as increment, which may have different meanings on different types

it is *not* defined as "is always the same as +=1 "

so it is definetly not a bug IHMO

the documentation issue is still valid 

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

"Incrementing or decrementing booleans has no effect."