<?php

class Member extends Model implements MemberInterface
{
    //...
    
  
    public function incrementPostCount()
    {
        $this->{self::ATTR_POST_COUNT}++;
        $this->save();
    }

    public function decrementPostCount()
    {
        if ($this->getPostCount() > 0) {
            $this->{self::ATTR_POST_COUNT}--;
            $this->save();
        }
    }
    
    // ...
}