From https://www.php.net/manual/en/migration81.incompatible.php:

Several resources have been migrated to objects. Return value checks using is_resource() should be replaced with checks for false.
The LDAP functions now accept and return, respectively, LDAP\ResultEntry objects instead of ldap result entry resources.
The LDAP functions now accept and return, respectively, LDAP\Connection objects instead of ldap link resources.
The LDAP functions now accept and return, respectively, LDAP\Result objects instead of ldap result resources.
The LDAP functions now accept and return, respectively, LDAP\ResultEntry objects instead of ldap result entry resources.

See also php/php-src#6770