A PHP mysql operation class

//Database operation class
class db
{
 //Data saving variables after SQL execution;
 var $db;
 //Read or set current data Position
 var $position=0;
 //Execute the SQL statement and save the result as a db variable;
 function sub_sql($str)
 {
 global $prefix;//Global Function, table prefix
 return str_replace("#@__",$prefix,$str);
 }
 function Sql($str)
 {
 $str=$this->sub_sql($str);
 $result = mysql_query($str);
 $i=0;
 while($row = mysql_fetch_array($result))
 {
 $str_array[$i]=$row;
 $i++;
 }
 if(empty($ str_array))
 {
 $str_array=array();
 }
 $this->db=$str_array;
 }
 //Read a piece of data and move the data back one bit, and return if the data is empty is null;
 function Get_One()
 {
 $re=empty($this->db[$this->position])?null:$this->db[$this->position];
 $this->position=$re?$this->position+1:$this->position;
 return $re;
 }
 //Judge whether the data has been read to the end
 function Judge()
 {
 $re=empty($this->db[$this->position])?true:false;
 return $re;
 }
 //Get the number in db
 function Get_Num()
 {
 return count($this->db);
 }
 //Update the data in the database, $t is the table name, $v format is the array format, the superscript is the field name, and the subscript is the data; $w is The superscript of the condition is the field name and the subscript is the data. $p is the condition. 0 is the equal sign, 1 is greater than, -1 is less than;
 function Set_Updata($t,$v,$w,$p=0)
 {
 $this->Sql($t);
 $v_str="";
 $w_str="";
 $f="";
 foreach($v as $key=>$vaule)
 {
 if (!is_numeric($key))
 {
 if(empty($v_str))
 {
 $v_str=htmlspecialchars($key)."='".htmlspecialchars($vaule)."'";
 }else
 {
 $v_str=$v_str.",".htmlspecialchars($key)."='".htmlspecialchars($vaule)."'";
 }
 }
 }
 switch($p)
 {
 case 0 :
 $f="=";
 break;
 case 1:
 $f=">";
 break;
 case -1:
 $f="<";
 break;
 }
 if( !empty($f))
 {
 foreach($w as $key=>$vaule)
 {
 if(!is_numeric($key))
 {
 if(empty($v_str))
 {
 $ w_str=htmlspecialchars($key).$f.htmlspecialchars($vaule)."'";
 }else
 {
 $w_str=$w_str.",".htmlspecialchars($key).$f.htmlspecialchars($vaule )."'";
 }
 }
 }
 }
 $sql="UPDATE ".$t." SET ".$v_str." where ".$w_str;
 return $result = mysql_query($sql);
 }
 //Delete a piece of data. $w is the condition. The superscript is the field name and the subscript is the data. $p is the condition. 0 is the equal sign, 1 is greater than, -1 is less than;
 function Set_Del($t,$w, $p=0)
 {
 $this->sub_sql($t);
 $w_str="";
 $f="";
 switch($p)
 {
 case 0:
 $f=" =";
 break;
 case 1:
 $f=">";
 break;
 case -1:
 $f="<";
 break;
 }
 if(!empty($f) )
 {
 foreach($w as $key=>$vaule)
 {
 if(!is_numeric($key))
 {
 if(empty($v_str))
 {
 $w_str=htmlspecialchars($key ).$f.htmlspecialchars($vaule)."'";
 }else
 {
 $w_str=$w_str.",".htmlspecialchars($key).$f.htmlspecialchars($vaule)."'";
 }
 }
 }
 }
 $str="DELETE FROM ".$t." WHERE ".$w_str;
 return $result = mysql_query($str);
 }
    function Add($t,$v)
    {
        $this->sub_sql($t);
        $k_str="";
        $v_str="";
        foreach($v as $key=>$vaule)
        {
            if(!is_numeric($key)){
                if(empty($k_str))
                {
                    $k_str=htmlspecialchars($key);
                    $v_str="'".htmlspecialchars($vaule)."'";
                }else
                {
                    $k_str=$k_str.",".htmlspecialchars($key);
                    $v_str=$v_str.","."'".htmlspecialchars($vaule)."'";
                }
            }
        }
        $str="INSERT INTO ".$t."(".$k_str.")"."value(".$v_str.")";
        return $result = mysql_query($str);
    }
}
?>