PHP字符串经典函数

<?    
//------------------------    
// PHP内置字符串函数实现     
//------------------------    
   
//字符串长度    
function strlen ($str)    
{    
        if ($str == '' ) return 0;    
        $count =  0;    
        while (1){    
                if ( $str[$count] != NULL){    
                         $count++;    
                        continue;    
                }else{    
                        break;    
                }    
        }    
        return $count;    
}    
   
//截取子串    
function substr($str, $start,  $length=NULL)    
{    
        if ($str== '' || $start>strlen($str )) return;    
    &