if($a): 6.3749399185181 (error reporting to log) -avoid this no matter what haha
if($a): 0.35815000534058 (no error reporting) -almost 10 times as slow as isset()
isset($a): 0.039632081985474
isset($a) && $a: 0.046839952468872
doempty($a): 0.039687871932983
!empty($a) && $a: 0.053149938583374

Where !empty($somevar) is a dropin replacement for if ($somevar) to avoid declaration errors.
You should always define, typecast, and test your variables appropriately to ensure your data is accurate, as opposed to relying solely on !empty()
eg: if (null === $somevar) or if (0 === $somevar)
If an undefined variable notice is generated. Fix the notice as it represents a logic error in the code and is potentially harmful to your application.
There are no excuses for lazy/bad coding practices.

Additionally until 5.5 empty could only be used with variables, making things like if (empty(trim($var))) resulted in an error.

<?php
function doNotice() {
if($a) echo "AH!";
}

function doIsset() {
if(isset($a)) echo "AH!";
}

function doBoth() {
if(isset($a) && $a) echo "AH!";
}

function doEmpty() {
if(!empty($a)) echo "AH!";
}

$t = microtime(true);
for($i = 0; $i < 1000000; $i++)
doNotice();
echo 'if($a): '.(microtime(true)-$t)."\n";

$t = microtime(true);
for($i = 0; $i < 1000000; $i++)
doIsset();
echo 'isset($a): '.(microtime(true)-$t)."\n";

$t = microtime(true);
for($i = 0; $i < 1000000; $i++)
doBoth();
echo 'isset($a) && $a: '.(microtime(true)-$t)."\n";

$t = microtime(true);
for($i = 0; $i

Which gives the following results:

if($a): 0.63301992416382
isset($a): 0.26364302635193
isset($a) && $a: 0.27497410774231
empty($a): 0.26639103889465

Even this is slightly faster:

isset($a); // Just to make sure it is declared.
if($a) { }