Objects are Passed by Reference

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Start another battle - how about 3 CloakShape fighters against 4 RZ-1 A-wing interceptors. Behind the scenes: each ship has a strength. The battle() function uses this as the ship's health, and as they battle each other, that health gets lower and lower until one hits zero.

We need to add a new feature: after the battle: display the final health of the battling ships. One will be zero or negative, but how much health did the other have left?

In battle(), those "ship health" variables are not returned in BattleResult. So we don't have access to this information. We could add it to BattleResult, but I want to do something more interesting.

After fighting a battle, let's update the strength of each ship with their new health: like $ship1->setStrength($ship1Health) and the same for $ship2:

... lines 1 - 9
public function battle(Ship $ship1, $ship1Quantity, Ship $ship2, $ship2Quantity)
{
... lines 12 - 16
while ($ship1Health > 0 && $ship2Health > 0) {
... lines 18 - 31
// now battle them normally
$ship1Health = $ship1Health - ($ship2->getWeaponPower() * $ship2Quantity);
$ship2Health = $ship2Health - ($ship1->getWeaponPower() * $ship1Quantity);
}
// update the strengths on the ships, so we can show this
$ship1->setStrength($ship1Health);
$ship2->setStrength($ship2Health);
... lines 40 - 56
}
... lines 58 - 66

After all, in real life - if a $ship is almost defeated, it's probably pretty broken - so it's $strength should reflect that.

Check this out by dumping $ship1->getStrength() and $ship2->getStrength() and die. Refresh! We have -14 and 116, 130 and 0 and so on.

Ok, working nicely, and that's simple. Actually, we just did something really important. Until now, this function has only read data from our ships. But now, we've changed those objects. In other words, in battle.php, we start with two Ship objects and pass them into battle():

106 lines battle.php
... lines 1 - 25
$ship1 = $ships[$ship1Name];
$ship2 = $ships[$ship2Name];
$battleManager = new BattleManager();
$battleResult = $battleManager->battle($ship1, $ship1Quantity, $ship2, $ship2Quantity);
... lines 32 - 106

Once that finishes running, those same two objects are different now: their data has changed.

This is totally different than how arrays work: if $ship1 were an array, and the battle() function changed one of its keys internally, that would have no effect here: $ship1 would still be the same array with the same original values.

Objects are passed by reference: it means that there is only one $ship1 object in existence and when we pass it to a function, we're passing that one object. But when you pass an array or a string to a function, you're actually passing a copy of the original value. If that value changes inside the function, it has no affect on the original variable.

Some of you may be familiar with adding an & symbol before an argument: this does the same thing: it makes that argument pass by reference. For objects, that's not needed, because this is always true.

The takeaway is that if you change an object, you're changing that object everywhere. To prove this, take our $ship1 and $ship2 - which are not returned by the battle() function - and add a new section that prints the finished strength. Add a dl element to make them a little pretty:

106 lines battle.php
... lines 1 - 33
<html>
... lines 35 - 53
<body>
<div class="container">
... lines 56 - 67
<div class="result-box center-block">
... lines 69 - 88
<h3>Remaining Strength</h3>
<dl class="dl-horizontal">
... lines 91 - 94
</dl>
</div>
... lines 97 - 102
</div>
</body>
</html>

First, echo $ship1->getName() and then $ship1->getStrength():

106 lines battle.php
... lines 1 - 33
<html>
... lines 35 - 53
<body>
<div class="container">
... lines 56 - 67
<div class="result-box center-block">
... lines 69 - 88
<h3>Remaining Strength</h3>
<dl class="dl-horizontal">
<dt><?php echo $ship1->getName(); ?></dt>
<dd><?php echo $ship1->getStrength(); ?></dd>
... lines 93 - 94
</dl>
</div>
... lines 97 - 102
</div>
</body>
</html>

Do the same thing for $ship2:

106 lines battle.php
... lines 1 - 33
<html>
... lines 35 - 53
<body>
<div class="container">
... lines 56 - 67
<div class="result-box center-block">
... lines 69 - 88
<h3>Remaining Strength</h3>
<dl class="dl-horizontal">
<dt><?php echo $ship1->getName(); ?></dt>
<dd><?php echo $ship1->getStrength(); ?></dd>
<dt><?php echo $ship2->getName(); ?></dt>
<dd><?php echo $ship2->getStrength(); ?></dd>
</dl>
</div>
... lines 97 - 102
</div>
</body>
</html>

We're missing auto-complete because we have some bad PHPDoc somewhere. We'll fix that in a bit.

Time to try it! Since objects are passed by reference, we should see the new, modified strength values - not the originals. Absolutely perfect.

Now let's get really wild and start fetching our ships from a database.

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