%t：表名占位符，DB::fetch_first("SELECT * FROM %t",array('common_member'));

%d：数值占位符，DB::fetch_first("SELECT * FROM %t WHERE uid=%d",array('common_member',1));会自动执行intval

%s：字符占位符，DB::fetch_first("SELECT * FROM %t WHERE username=%s",array('common_member','管理员'));会自动执行addslashes，转义特殊字符

%n：DB::fetch_all("SELECT * FROM %t WHERE uid IN(%n)",array('common_member',array(1,2,3)));

%f：同%d类似，只是浮点数;

%i：原样输出，DB::fetch_first("SELECT * FROM %t WHERE username='%i'",array('common_member','管理员'));