date from Strings with fixed start & end date

test.php

<?php
$festivalStart = '2019-07-29';
$festivalEnd = '2019-08-04';

$germanDays = ['Montag', 'Dienstag', 'Mittwoch', 'Donnerstag', 'Freitag', 'Samstag', 'Sonntag'];
$englishDays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'];

function get_begin_N_end_from_time_span($timeSpanString)
{
    preg_match('/(?P<begin>\w+).{0,3} (?P<end>\w+)/', $timeSpanString, $matches);

    return $matches;
}

$periodSingle = 'Donnerstag';


function handle_single_period($periodSingle) 
{
    $germanDays = ['Montag', 'Dienstag', 'Mittwoch', 'Donnerstag', 'Freitag', 'Samstag', 'Sonntag'];
    $englishDays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'];
    $festivalStart = '2019-07-29';
    $festivalEnd = '2019-08-04';

    $translatedPeriod = str_replace($germanDays, $englishDays, $periodSingle);
    
    $startObject = new DateTime($festivalStart);
    $festivalStartDay = $startObject->format('l');
    
    $indexPeriodSingle = array_search($translatedPeriod, $englishDays);
    $indexFestivalStartDay = array_search($festivalStartDay, $englishDays);
    $difference = $indexPeriodSingle - $indexFestivalStartDay;
    
    $periodSingleDateObject = $startObject->modify("+{$difference} days");
    return $periodSingleDateObject->format('Y-m-d');

}    

// echo handle_single_period($periodSingle);

$periodTimeSpan = 'Montag - Sonntag';
function handle_time_span($timeSpanString) 
{
    $timeSpan = get_begin_N_end_from_time_span($timeSpanString);

    $germanDays = ['Montag', 'Dienstag', 'Mittwoch', 'Donnerstag', 'Freitag', 'Samstag', 'Sonntag'];
    $englishDays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'];
    $festivalStart = '2019-07-29';
    $festivalEnd = '2019-08-04';

    $endDateObject = new DateTime($festivalEnd);
    $festivalEndDay = $endDateObject->format('l');

    $translatedEndDay = str_replace($germanDays, $englishDays, $timeSpan['end']);
    
    $indexPeriodEnd = array_search($translatedEndDay, $englishDays);
    $indexFestivalEnd = array_search($festivalEndDay, $englishDays);
    $differenceEnds = $indexFestivalEnd - $indexPeriodEnd;
    
    $userEndDate = $endDateObject->modify("-{$differenceEnds} days");
    
    $startDateObject = new DateTime($festivalStart);
    $festivalStartDay = $startDateObject->format('l');
    
    $translatedStartDay = str_replace($germanDays, $englishDays, $timeSpan['begin']);

    $indexPeriodStart = array_search($translatedStartDay, $englishDays);
    $indexFestivalStart = array_search($festivalStartDay, $englishDays);
    $differenceStarts = $indexPeriodStart - $indexFestivalStart;
    $userStartDate = $startDateObject->modify("+{$differenceStarts} days");

    return ['startDate' => $userStartDate->format('Y-m-d'), 'endDate' => $userEndDate->format('Y-m-d')];
}

var_dump(handle_time_span($periodTimeSpan));