php字符串反转 面试中经常遇到的问题

php提供了用于字符串反转的函数strrev()  
$str = 'abcdef';
echo strrev($str);  
2.对于包含中文的多字节字符串需要用到mb_substr()   
$str = '字符串反转'; 
function rev($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)   
3.算法实现 首位交换
$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;