Game Design: Calculating Experience Points earned after a battle

I'm developing a simple text battle game and having some issues calculating how much XP should be earned after a battle, theres a few factors that i want to consider:

1) Players should of course earn good XP if they beat someone of the same level

2) Players should earn great XP if they beat someone of a higher level

3) Players should earn poor XP if they defeat someone of a lower level.

4) The above should scale nicely, i.e. if player A is level 10 and battles Player B who's level 9 or 11 the difference shouldn't be huge (they would of course gain more XP for defeating the level 11 character but we wouldn't expect t开发者_开发问答his to be huge), but if he was to battle a level 5 or 15 this difference should be huge.

Just having trouble calculating something that works well in all scenarios, currently my XP table looks like the below:

Current Level   XP Needed   Increase from Last Level
1   30  0
2   65  35
3   106 41
4   154 48
5   210 56
...
10  672 123

In PHP my code looks like the below to calculate the XP Needed:

$offset = 30;
$multiplier = 1.17;
$base = $prevXp * $multiplier;

$xp = $base + $offset;

I've tried a few different things but i haven't been impressed with any of them, I'd like to work with calculating the difference in XP between the two players and then using some kind of multiplier but i haven't been impressed with the results so far.

Any help / input would be appreciated.

Here is the fight method I used for a FB game I ditched. You might find it useful.

public function fight(Player $attacker, Player $defender){
      $return = 'Attacking '.$defender->getName().'
';
      // if the defener has greater def than attackers attack, repel the attack
      // if the attacker has greater attack then the denfenders defense, penetrate
      if($attacker->getAttack() > $defender->getDefense()){
        $attacker->takeTemper(1);
        // penetration -- that's what she said!!!
        // figure out how much to damage each player
        $dmgDefender = rand(5, 25);
        $defender->takeHP($dmgDefender);
        $dmgAttacker = rand(0, 10);
        // figure out the reward
        // give between 1 and 5 XP and 10% of the defenders money
        $gainedXP = rand(1, 5);
        $attacker->addXP($gainedXP);
        $gainedMoney = floor($defender->getMoney() * .1);
        $attacker->addMoney($gainedMoney);
        $defender->takeMoney($gainedMoney);
        $return .= 'You won the fight and gained '.$gainedXP.' XP and $'.$gainedMoney.'
';
        if($dmgAttacker > 0){
          $return .= 'Unfortuantely you took '.$dmgAttacker.' damage in the fight.  The good news is you did '.$dmgDefender.' damage to '.$defender->getName().'
';
          $attacker->takeHP($dmgAttacker);
        }
        // check if the player leveled
        $nextLevelXP = $this->_level->levelXP( $attacker->getLevel() + 1 );
        if( ( $attacker->getXP() >= $nextLevelXP ) ){
          $gainedFounderPoints = $this->gainLevel($attacker);
          $return .= 'You gained a level';
          if($gainedFounderPoints > 0)
            $return .= ' and '.$gainedFounderPoints.IMG_FOUNDER_POINT;
          $return .= '
';
        }
      } else {
        // attack deflected
        $return .= 'The attack was deflected by '.$defender->getName().'
';
        // reward defender -- how about 10% of the attackers money?
        $reward = ceil($attacker->getMoney() * .1);
        if($reward > 0){
          // demerit attacker
          $attacker->takeMoney($reward);
          $defender->giveMoney($reward);
          $return .= 'You lost $'.$reward.'
';
        } else {
          // the attacker is broke ... take his XP!!! -- 1 percent
          $reward = ceil($attacker->getXP() * .01);
          $attacker->takeXP($reward);
          $defender->addXP($reward);
          $return .= 'You lost '.$reward.' XP
';
        }
      }
      // save data
      $attacker->save();
      $defender->save();

      // output
      return $return;
    }

this is kind of simple but it fits your criteria

xpGain = getbasexp(); //whatever you were doing before
xpGain *= opponentxp * 1.0 / playerxp;