break is language construct and cannot be used in such expressions just becase PHP can't compute the value of such expression. 

On the other hand, die() is just a function, which returns a result value.

couple of example:
if ($foo == $var || break) //wrong. break returns nothing and PHP cannot compute expression's value.
if ($foo == $var || echo) //wrong. echo returns nothing
if ($foo == $var || strlen('some')) // ok
$foo == 1 OR echo 1; //wrong
$foo == 1 OR print 1; //correct. print() always return true.

Thanks, I understand.
I rewrite my code to:
if ( !($res = some_func()) ) break;

But this line looks more nicely and it is a pity that she does not work:
$res = some_func() or break;

Thanks!

Not a bug