- array_map: documentation ( source)
- implode: documentation ( source)
<?php
$days = [
1 => 'Sunday',
2 => 'Monday',
3 => 'Tuesday',
4 => 'Wednesday',
5 => 'Thursday',
6 => 'Friday',
7 => 'Saturday'
];
$array = [7,3,4];
$result = array_map(function ($a) use($days) { return $days[$a]; }, $array);
echo "Day in name= ".implode(',', $result);