PHP - 计算一个数组中的值出现在另一个数组中的次数，然后创建一个包含这些数字的数组

I have two arrays, both in Y-m-d format. One array is the amount of days that has passed in the current month, and one array is a list of when every user on my app has been created in Y-m-d format. I want to create a third array which counts the amount of times a user was created on a certain day in my days-of-month array, then puts that count into the third array.

My code so far is this:

$daysInCurrentMonth = date('t');
$daysPast = date('j');
$month = date('m');
$year = date('y');

$participants = DB::table('participants')->get();
$createdAt = $participants->pluck('created_at');

foreach($createdAt as $created)
{
    $dateTimes[] = Carbon::createFromFormat('Y-m-d H:i:s', $created)->format('Y-m-d');
}

$dates = $this->generateDateRange(Carbon::parse($year.'-'.$month.'-01'), Carbon::parse($year.'-'.$month.'-'.$daysPast));

The $dates array has:

array (size=5)
  0 => string '2018-10-01' (length=10)
  1 => string '2018-10-02' (length=10)
  2 => string '2018-10-03' (length=10)
  3 => string '2018-10-04' (length=10)
  4 => string '2018-10-05' (length=10)

The $dateTimes array has:

array (size=6)
  0 => string '2018-09-21' (length=10)
  1 => string '2018-09-24' (length=10)
  2 => string '2018-09-24' (length=10)
  3 => string '2018-10-02' (length=10)
  4 => string '2018-10-04' (length=10)
  5 => string '2018-10-04' (length=10)

I want the third array to loop through all the days in $dates and for every date there is no tally put a 0, so given the above data my array would look like this:

$matches = [0, 1, 0, 2, 0]

I've tried a bunch of PHP array functions but I was wondering if someone could quickly help me out.

Try something like this:

// Groups $dateTimes by number of occurrences 
$registrations = array_count_values($dateTimes);

$matches = [];
foreach ($dates as $key => $date) {
    $matches[$key] = isset($registrations[$date]) ? $registrations[$date] : 0;
}

If you're using PHP 7.x you can use $matches[$key] = $registrations[$date] ?? 0; to shorten it a bit.

// the final array that holds umber of times user was created.
// as a key value pair ( 2018-01-01 => 5 )
$counts = [];

foreach ($dates as $date) {

    $count = 0;

    foreach ($dateTimes as $dateTime) {
        if ($date == $dateTime) $count++;
    }

    array_set($counts, $date, $count);
}