My setup:
php3.0RC3
linux RH5.0
mysql-3.21.29 (compiled in mysql support)
no php3.ini file - defaults compiled in)

A "--" operator doesn't work when the variable is set in URL
(didn't tested POST method, only GET). At the same time, however,
the "++" operator works.

Script "decrtest.html"
<?
#$a=0;
echo "$a"; $a++; echo "$a"; $a++; echo "$a";
$a--; echo "$a";$a--;echo "$a";
$a++; echo "$a";$a++; echo "$a";
?>

Results:
http://localhost/decrtest.html
121012 (this is OK, the first $a=="" so it's not visible)

http://localhost/decrtest.html?a=5
5677789 (BUG - no decrementing)

After uncommenting #$a=0:
http://localhost/decrtest.html
0121012