- min: documentation ( source)
<?php
$startYear = 2014;
$currentYear = 2015;
$lastTaken = 2014;
$recurrenceYears = 3;
$useYear = MIN($currentYear, $lastTaken);
$nextRequirement = $useYear + ($recurrenceYears - (($useYear - $startYear) % $recurrenceYears));
echo $nextRequirement;