$bm = microtime_float();
for ($a = 0; ++$a < 1000000;) {
	$b;
}

echo microtime_float() - $bm;

0.13535499572754

$bm = microtime_float();
for ($a = 0; ++$a < 1000000;) {
	isset($b);
}

echo microtime_float() - $bm;

0.12668704986572

$bm = microtime_float();
for ($a = 0; ++$a < 1000000;) {
	!empty($b);
}

echo microtime_float() - $bm;

0.14148497581482

$bm = microtime_float();
for ($a = 0; ++$a < 1000000;) {
	$a;
}

echo microtime_float() - $bm;

0.096091032028198

$bm = microtime_float();
for ($a = 0; ++$a < 1000000;) {
	isset($a);
}

echo microtime_float() - $bm;

0.14264321327209

$bm = microtime_float();
for ($a = 0; ++$a < 1000000;) {
	!empty($a);
}

echo microtime_float() - $bm;

0.15884304046631

Nota: Este servidor corre PHP 4, por lo que se definió microtime_float como en el ejemplo 447