The $entry parameter can be an array of values for an attribute. Just be careful that your array's indices are numerically contiguous. For example, when using this $entry array, ldap_modify will fail with little explanation:
$entry = array( 0 => 'foo', 2 => 'bar' );
While this one will work just fine:
$entry = array( 0 => 'foo', 1 => 'bar' );
Hope this helps someone out.