$newArrOne = [];//初始化一个新的数组
 $newStrOne = '';//初始化一个新的字符串
 $newArrOne = str_split($str);
 $arrCount = count($newArrOne);
 for ($i=0; $i < $arrCount; $i++) {
 $newStrOne.=$newArrOne[$i];
 }
 echo "<pre>";
 print_r($newStrOne);
 echo "</pre>";

$newStrTwo = '';//初始化一个新的字符串
 $arrCountTwo = strlen($str);
 for ($i=1; $i <= $arrCountTwo; $i++) {
 $newStrTwo.=substr($str, -$i, 1);
 }
 echo "<pre>";
 print_r($newStrTwo)."\n";
 echo "</pre>";

$newStrThree = '';//初始化一个新的字符串
$arrCountThree = strlen($str);
for ($i = $arrCountThree; $i>=0;$i--) {
 @$newStrThree.=$str[$i];
}
echo "<pre>";
print_r($newStrThree)."\n";
echo "</pre>";