<?php

$tabel = [
    ['id' => 1, 'title' => 'Event 1', 'startdate' => '2019-03-22', 'enddate' => '2019-03-24'],
    ['id' => 2, 'title' => 'Event 2', 'startdate' => '2019-03-23', 'enddate' => '2019-03-27']
];

$output = [];

foreach($tabel as $row) {
    $period = new DatePeriod(new DateTime($row['startdate']), new DateInterval('P1D'), new DateTime($row['enddate'] . " +1 second"));

    /** @var DateTime $dateTime */
    foreach($period as $dateTime) {
        $output[$dateTime->format('Y-m-d')][] = $row['title'];
    }
}

var_dump(array_map(function(array $outputRow) {
    return implode(', ', $outputRow);
}, $output));