Reverse String in various ways

<?php

$sentence = "My name is Jon";
echo "<br>Main sentence is:<br>";
echo $sentence;
$rev1= strrev($sentence);
echo "<br>First Reversal:<br>";
echo $rev1;
echo "<br>2nd Reversal system: <br>";
$rev2= "";
for($i=1;$i<=strlen($sentence);$i++){
 
$rev2.=$sentence[$i * -1] ;
}

 
echo  $rev2 ;

$rev3="";
echo "<br>3nd Reversal system: <br>";

for($i=strlen($sentence);$i>=0;$i--){
$rev3.= $sentence[$i];
}
echo $rev3;
?>

Main sentence is:
My name is Jon
First Reversal:
noJ si eman yM
2nd Reversal system:
noJ si eman yM
3nd Reversal system:
noJ si eman yM