I just ran a simple benchmark. Here are the results (for 4.3.5-dev):

$var is undefined
--------------------------------------------------------------------------
if ($var)                    1075359033.35615200   1.53820896149    55.22%
--------------------------------------------------------------------------
if (isset($var))             1075359033.75500700   0.398854970932   14.32%
--------------------------------------------------------------------------
if (empty($var))             1075359034.16585300   0.410845994949   14.75%
--------------------------------------------------------------------------
if (isset($var) && $var)     1075359034.60332200   0.437469005585   15.70%
--------------------------------------------------------------------------

$var = ''; / $var = 0; / $var = 1;
--------------------------------------------------------------------------
if ($var)                    1075358938.68592300   0.353079080582   19.38%
--------------------------------------------------------------------------
if (isset($var))             1075358939.12858000   0.442656993866   24.29%
--------------------------------------------------------------------------
if (empty($var))             1075358939.56134500   0.432765007019   23.75%
--------------------------------------------------------------------------
if (isset($var) && $var)     1075358940.15466400   0.593318939209   32.56%
--------------------------------------------------------------------------

The tests are nearly identical under PHP5 except the 'if ($var)' test
for '$$var = ''; / $var = 0; / $var = 1;' was about 2% slower.