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

echo microtime_float() - $bm;

0.14294600486755

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

echo microtime_float() - $bm;

0.19136905670166

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

echo microtime_float() - $bm;

0.21312093734741

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

echo microtime_float() - $bm;

0.10635614395142

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

echo microtime_float() - $bm;

0.18292498588562

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

echo microtime_float() - $bm;

0.20766806602478

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