strlen()完全不用php内部函数如何实现？

<code>function mystrlen($str){
   $i = 0;
   while(1)if(!isset($str[$i++]))return $i-1;
}
</code>

<code>function mystrlen($str){
   $i = -1;
   while(1) if(!isset($str[++$i])) return $i;
}
</code>

<code>function mystrlen($str){
   $size = 1024;//每次试探1024的范围
   $count = 0;//试探并且满足长度的次数
   $length = $size * $count - 1;//当前可以确定字符串具有的长度
   while( isset( $str[ $length + $size ] ) ){//依次试探1023,2047,3071,4095……
      $count = $count + 1;
      $length = $length + $size;
   }
   $low = 0;//二分查找下边界
   $high = $size - 1;//二分查找上边界
   while($low <= $high) {
     $mid = floor(($low + $high) / 2);
     if( isset( $str[$length + $mid] ) && !isset( $str[$length + $mid + 1] )){
        return $length + $mid + 1;
     }
     if( !isset( $str[$length + $mid] )){
       $high = $mid - 1;
     }
     if( isset( $str[$length + $mid] )){
       $low = $mid + 1;
     }
  }
}
</code>