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

echo microtime_float() - $bm;

0.11579608917236

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

echo microtime_float() - $bm;

0.18190312385559

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

echo microtime_float() - $bm;

0.20620894432068

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

echo microtime_float() - $bm;

0.11346411705017

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

echo microtime_float() - $bm;

0.18158197402954

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

echo microtime_float() - $bm;

0.22020602226257

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