<?php
 
function sampling($days, $size, $combinations = array()) {
 
    # if it's the first iteration, the first set 
    # of combinations is the same as the set of characters
    if (empty($combinations)) {
        $combinations = array_map(function($day) { return array($day); }, $days);
    }
 
    # we're done if we're at size 1
    if ($size == 1) {
        return $combinations;
    }
 
    # initialise array to put new values in
    $new_combinations = array();
 
    # loop through existing combinations and character set to create strings
    foreach ($combinations as $combination) {
        foreach ($days as $day) {
            if (!in_array($day, $combination)) {
                $new_combination = $combination;
                $new_combination[] = $day;
                $new_combinations[] = $new_combination;
            }
        }
    }
 
    # call same function again for the next iteration
    return sampling($days, $size - 1, $new_combinations);
}
 
$days = array('Monday', 'Tuesday', 'Thursday');
 
print_r(sampling($days, 2));
print_r(sampling($days, 3));

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