在foreach中修改数组并理解原因的问题

Question

I always did something like

e.g. from my last project in Laravel

foreach( $sections as $section )
{
    $section->date = Carbon::parse($section->date)->diffForHumans();
}

And it worked well. Never problem with that, array was modified as I wanted

But now I did this

$events = $this->events;
        foreach($events as $event)
        {
            $date = new \DateTime($event['date']);
            $event['date'] = (int)$date->format('d');
            echo $event['date'];
            var_dump($event);
        }
        var_dump($events);
        $this->events = $events;

And it doesn't work as I want, different values inside loop and outside loop.

I was looking what is bad and I found what it is about, to give & or do foreach $key => $value...

But it would be okay.

But it isn't.

Because I know. I remember. I did many times just something like

foreach($a as $b)
{
    $b = .......
}

And it worked. But now it isn't.

So I don't understand. But I have to and want to.

I was searching for answer, but can find it.

Why for one time it works for me like that, and other time doesn't work and I need apersand or key => value playing?

Please give me understanding <3

Why one time it is working but another not.

Answer 1

foreach iterates over a copy of the array, so modifications to the array elements are only made to that temporary copy unless you create a reference:

foreach($a as &$b) {
    $b = 'something';
}

Or modify the original array by key:

foreach($a as $k => $b) {
    $a[$k] = 'something';
}

For an array of objects, the array is a copy but the objects in the copy are references to the original array. So modifications to the objects in the copy are reflected in the original array:

foreach($a as $b) {
    $b->var = 'something';
}