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

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

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