isset / strlen

$a = 'fdsafdsafdsafdsafdsafdsafdsafdsafdsafdsafdsafdsafdsafdsa';

$st = getmicrotime();
for ($i = 0; $i < $cnt; $i++) {
	if (!isset($a[0])) {
		//echo 'a';
	}
}
print (getmicrotime() - $st)."\n";

$st = getmicrotime();
for ($i = 0; $i < $cnt; $i++) {
	if (!strlen($a)) {
		//echo 'a';
	}
}
print (getmicrotime() - $st)."\n";


# isset
0.10243892669678

# strlen
6.4311540126801