php各种常用的算法

// 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;  
        if (($length!=NULL) && ( $start>0) && ($length> strlen($str)-$start)) return; 
        if (( $length!=NULL) && ($start< 0) && ($length>strlen($str )+$start)) return; 
        if ($length ==  NULL) $length = (strlen($str ) - $start);  
          
        if ($start <  0){  
                for ($i=(strlen( $str)+$start);