foreach does not trigger this notice. Please check the reported line number carefully. It must involve something like "$array[$offset]".

Hm, right. I guess I need to explicitly skip the "count" element now which is part of the $result array.

foreach (array_slice($entries, 1) as $e) { .. }

appears to work. Not sure if this is the cleanest solution though. WDYT?

Could you please post the relevant code (and show the reported
line number)?

$result = ldap_search($con, $dn, $filter, $justthese);
$entries = ldap_get_entries($con, $result);

foreach ($entries as $e) {
    if ($e[$this->uidNameAttribute][0] == '') {
        continue;
    }
    $principal = array('uri' => 'principals/' . strtolower($e[$this->uidNameAttribute][0]),);
    foreach ($this->fieldMap as $key => $value) {
        if (isset($e[$value['ldapProperty']])) {
            $principal[$key] = $e[$value['ldapProperty']][0];
        }
    }
    $principals[] = $principal;
}

Let's close this. This is an issue that needs to be solved in our code.
The issue was just hidden with previous PHP releases. We need to handle the 'count' array entry explicitly.
Not sure yet, if array_slice() is the proper solution. I guess there is no guarantee that the 'count' entry will always be the first entry in the result array.

@nikic: I wonder, should the example at https://www.php.net/manual/en/ldap.examples-controls.php be updated accordingly?

E.g. Example #5 uses

    $entries = ldap_get_entries($link, $result);
    foreach ($entries as $entry) {
        echo "cn: ".$entry['cn'][0]."\n";
    }

With PHP 7.4 this will now trigger the aforementioned notice afaics.