PHP Check empty string perfomance test

<?php

$sa = array('some string', '', 'test1', 'test2', '');
$length = count($sa);
$b = false;
$limit = 1000000;
$times = $limit*$length;

$start = microtime(true);

for($i = 0; $i < $limit; $i++){
    foreach($sa as $s){
        $b = (strlen($s) > 0);
    }
}

echo 'strlen() > 0 used '.$times.' times in ' . (microtime(true) - $start) . ' msec' . "\n\n";

$start = microtime(true);

for($i = 0; $i < $limit; $i++){
    foreach($sa as $s){
        $b = ($s !== '');
    }
}

echo 'var !== \'\' used '.$times.' times in ' . (microtime(true) - $start) . ' msec' . "\n\n";

$start = microtime(true);

for($i = 0; $i < $limit; $i++){
    foreach($sa as $s){
        $b = !empty($s);
    }
}

echo '!empty() used '.$times.' times in ' . (microtime(true) - $start) . ' msec' . "\n\n";