$str = 'abcdef';
echo strrev($str);

$str = '字符串反转';
function rev($str, $encoding = 'utf-8'){
 $len = mb_strlen($str);
 $result = '';
 for ($i = $len-1; $i>=0; $i--){
  $result.= mb_substr($str,$i,1,$encoding);
 }
 return $result;
}
echo rev($str);

$str = 'abcdefg';
$len = strlen($str);
$times = $len/2;
for($i = 0;$i = $times; $i++ ){
 $tmp = $str[$i];
 $str[$i] = $str[$len-$i-1];
 $str[$len-$i-1] = $tmp;
}
echo $str;