- max: documentation ( source)
- min: documentation ( source)
<?php
$datetimeStart1 = new DateTime('2017-05-16');
$datetimeEnd1 = new DateTime('2017-07-19');
$datetimeStart2 = new DateTime('2017-07-12');
$datetimeEnd2 = new DateTime('2017-07-22');
// following http://stackoverflow.com/questions/325933/determine-whether-two-date-ranges-overlap
if ($datetimeStart1 < $datetimeEnd2 && $datetimeEnd1 > $datetimeStart2) {
echo min($datetimeEnd1,$datetimeEnd2)->diff(max($datetimeStart2,$datetimeStart1))->days+1;
} else {
echo 'no overlap';
}