References: Assign by Reference

Normally, $b = $a copies the value. With a reference, both names point to the same value, so a change through one name shows up in the other.

You create a reference with &.

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<?php
$a = 5;
$b = &$a;   // $b is another name for $a

$b = 99;

echo "a is $a, b is $b";

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a is 99, b is 99

Think of it as two name tags on one box.

Where references are useful

The most common use is a function that must change the caller’s variable.

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<?php
function addTax(float &$price): void
{
    $price = $price * 1.18;
}

$total = 100.0;
addTax($total);

echo $total;

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118

Without the &, the function would change only its own copy. You will see this again in Passing by Reference.

References in foreach

You can change array items while looping by using &$item.

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<?php
$prices = [10, 20, 30];

foreach ($prices as &$price) {
    $price = $price * 2;
}
unset($price); // important: break the reference

print_r($prices);

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Array
(
    [0] => 20
    [1] => 40
    [2] => 60
)