Is playing online poker considered forbidden (haram), knowing that it doesn't involve real money and is played for entertainment?

The game you mentioned is a type of forbidden gambling if it involves a wager. If it does not involve a wager, then there is a difference of opinion among scholars concerning it. Some divide gambling (maysir) into maysir of amusement (without money) and maysir of wagering (with money). Anything that distracts from the remembrance of Allah and from prayer is considered maysir. Therefore, if this game involves something forbidden, such as being played for money, or if it distracts from the remembrance of Allah and prayer, or causes enmity, or distracts from obedience, then it is forbidden. However, if it is free from such things, it is disliked due to its lack of benefit.