A simple comprehensive class for database connection and text caching

Nothing special, just encapsulate the database connection and text cache in a class

class Db{
 protected $_connect;
 protected $_db = Array();
 protected $_cache = Array();

 public function __construct($args){
 list($this->_db, $this->_cache) = $args;
 }
 protected function connect($db){
 $this->_connect = mysql_connect($db['hostname'],$db['username'],$db ['password']);
 mysql_set_charset('UTF8');
 mysql_select_db($db['databasename'],$this->_connect);
 }
 /**
 *Function: Get the data in the table and format and return the queried data. The return format is in array form!
 *$sql: The incoming SQL statement to be executed must and can only be SELECT
 *
*/
 public function fetch( $sql){
 $result = '';
 if(isset($this->_cache['expire'])){
 $name = md5(strtolower(str_replace(' ','',$sql)) );
 $dir = substr($name,0,2);
 $dir = $this->_cache['dir'].'/'.$dir;
 !is_dir($dir) && mkdir($ dir,0777);
 !is_dir($dir) && mkdir($dir,0777);
 $this->_cache['path'] = $dir.'/'.$name;
 if(is_file($ this->_cache['path']) && $this->check_expire()){
 $result = $this->get();
 }
 }
 if($result == ''){
 $data = $this->exec($sql);
 $result = Array();
 while($result[] = mysql_fetch_array($data,MYSQL_ASSOC)){} //Remove index
 mysql_free_result($data );
 array_pop($result);
 isset($this->_cache['expire']) && $this->write($result);
 }
 return $result;
 }
 /**
 *Function: Execute all SQL statements, but not including SELECT!
 *$sql: The incoming SQL statement to be executed cannot be SELECT
 *Return value: TRUE OR FALSE
*/
 public function exec($sql){
 if($this->_connect === null) $this->connect($this->_db); //Perform data link
 if( $result = mysql_query($sql, $this->_connect) ){
 return $result;
 }else{
 die("{$sql}Execution error: " . mysql_error());
 }
 }
 /**
 *Function: Execute database insertion statement, which can only be INSERT statement!
 *$v: The passed-in conditions to be executed are in array format, table represents the table to be inserted, row is the field, and value is the value to be inserted
 *Return value: mysql_insert_id() OR FALSE
 */
 public function insert($table,$field,$ignore = 0){
 $D = Array('field'=>'','val'=>'');
 foreach($field AS $key => $v){
 $D['field'] .= $key.',';
 $D['val'] .= "'{$this->escape($v)}',";
 }
 $D['field'] = rtrim($D['field'],',');
 $D['val'] = rtrim($D['val'],',');
 $ignore = $ignore > 0 ? 'IGNORE' : '';
 $sql = "INSERT {$ignore} INTO {$this->_db['perfix']}{$table}({$D[' field']}) VALUES({$D['val']})";
 if($this->exec($sql)){
 $insert_id = mysql_insert_id();
 return is_numeric($insert_id) ? $insert_id : TRUE;
 }else{
 return FALSE;
 }
 }
 public function update($table,$field){
 $D = Array('where'=>'','str'=> '');
 $index = 0;
 foreach($field AS $key => $v){
 $index == 0 ? $D['where'] = "{$key} = '{$this ->escape($v)}'" : $D['str'] .= "{$key} = '{$this->escape($v)}',";
 $index++;
 }
 $D['str'] = rtrim($D['str'],',');
 $sql = "UPDATE {$this->_db['perfix']}{$table} SET {$ D['str']} WHERE {$D['where']}";
 return $this->exec($sql);

 }
 public function delete($table,$field){
 $str = '';
 foreach($field AS $key => $v){
 $str = "{$key} = '{$v}'";
 }
 $sql = 'DELETE FROM '.$this ->_db['perfix'].$table.' WHERE '.$str.' LIMIT 1';
 return $this->exec($sql);
 }
 public function sum($table,$condition ){
 $totle = $this->fetch('SELECT COUNT(*) AS totle FROM '.$this->_db['perfix'].$table.' WHERE '.$condition);
 return $ totle[0]['totle'];
 }
 /**
 *Function: Filter the input special characters
 *$v: Parameters to be passed in for detection
 *Return value: Detected parameters
*/
 public function escape($v){
 return mysql_real_escape_string($v);
 }
 /*
 *Function: perform cache judgment
 */
 public function cache($name,$expire=100000000){
 $this->_cache['expire'] = $expire;
 return $this;
 }
 public function check_expire(){
 return ( filemtime($this->_cache['path']) + $this->_cache['expire']) > strtotime("now");
 }

 public function write($data){
 $ f = fopen($this->_cache['path'], 'w');
 if ($f) {
 flock($f, LOCK_EX);
 fseek($f, 0);
 ftruncate($ f, 0);
 $tmp = fwrite($f, serialize($data));
 if (!($tmp === false)) {
 $result = true;
 }
 fclose($f);
 }
 chmod($this->_cache['path'],0777);
 }
 public function get(){
 $f = fopen($this->_cache['path'], 'r' );
 $data = fread($f,filesize($this->_cache['path']));
 fclose($f);
 return unserialize($data);
 }
 public function delete_dir($ dir = ''){
 $dir = empty($dir) ? $this->_cache['dir'] : $dir;
 !is_dir($dir) && exit;
 $d = opendir($dir) ;
 $i = 0;
 while(($file = readdir($d)) !== false){
 $path = $dir.'/'.$file;
 if($i > 1) is_file ($path) ? unlink($path) : $this->delete_dir($path);
 $i++;
 }
 closedir($d);
 rmdir($dir);
 }
 public function __destruct() {
 isset($this->_connect) && mysql_close($this->_connect);
 }
}