- array_search: documentation ( source)
- array_keys: documentation ( source)
<?php
$users_emails = array('Spence', 'Matt', 'Marc', 'Adam', 'Paul');
$current = 'Paul';
$keys = array_keys($users_emails);
$ordinal = (array_search($current,$users_emails)+1);
$next = $users_emails[$ordinal];
echo $next;