- array_diff_assoc: documentation ( source)
- array_merge: documentation ( source)
- array_unique: documentation ( source)
- array_diff: documentation ( source)
- key: documentation ( source)
<?php
$first = Array(607,608,609,610);
$second = Array(607,608,609,800);
$Fcount = count($first);
$arr = array_merge($first, $second);
$arrc= key(array_diff($arr, array_diff_assoc($arr, array_unique($arr))));
echo $arrc;