I have been running some small tests in PHP on loops. I do not know if my method is good.

I have found that a inverse loop is faster than a normal loop.

I have also found that a while-loop is faster than a for-loop.

Setup

<?php

$counter = 10000000;
$w=0;$x=0;$y=0;$z=0;
$wstart=0;$xstart=0;$ystart=0;$zstart=0;
$wend=0;$xend=0;$yend=0;$zend=0;

$wstart = microtime(true);
for($w=0; $w<$counter; $w++){
    echo '';
}
$wend = microtime(true);
echo "normal for: " . ($wend - $wstart) . "<br />";

$xstart = microtime(true);
for($x=$counter; $x>0; $x--){
    echo '';
}
$xend = microtime(true);
echo "inverse for: " . ($xend - $xstart) . "<br />";

echo "<hr> normal - inverse: " 
        . (($wend - $wstart) - ($xend - $xstart)) 
        . "<hr>";

$ystart = microtime(true);
$y=0;
while($y<$counter){
    echo '';
    $y++;
}
$yend = microtime(true);
echo "normal while: " . ($yend - $ystart) . "<br />";

$zstart = microtime(true);
$z=$counter;
while($z>0){
    echo '';
    $z--;
}
$zend = microtime(true);
echo "inverse while: " . ($zend - $zstart) . "<br />";

echo "<hr> normal - inverse: " 
        . (($yend - $ystart) - ($zend - $zstart)) 
        . "<hr>";

echo "<hr> inverse for - inverse while: " 
        . (($xend - $xstart) - ($zend - $zstart))
        . "<hr>";
?>

Average Results

The difference in for-loop

normal for: 1.0908501148224
inverse for: 1.0212800502777

normal - inverse: 0.069570064544678

The difference in while-loop

normal while: 1.0395669937134
inverse while: 0.99321985244751
normal - inverse: 0.046347141265869

The difference in for-loop and while-loop

inverse for - inverse while: 0.0280601978302

Questions

My question is can someone explain these differences in results? And is my method of benchmarking been correct?