<?
  $i=10;
  print "hello " . $i-2 . " world";
?>

will output:   -2 world

("hello " . $i is evaluated into "hello 10", then 2 is subtracted (by casting "hello 10" into a 0, then subtracting 2).  Then "world" is appened, thus giving us "-2 world")

I think this might be way counter-intuitive to what should be happening: - (or all other operations) should be avaluated first, and concatenation last, producing "hello 8 world".

Or how about this example, which definately can confuse some people:

print "Hello is less than " . 1+strlen('hello') . " characters long.";

Workaround: use () to arrange things to be executed in proper order but that might not be obvious for someone who doesn't know about this situation.

PS:
Thanks PartyZan for finding this.