Yes, the PHP client can do it. A reindex API is just the combination of:
- Scan/scroll API to get all your data
- Bulk indexing to re-insert the data into a new index
The PHP client (and all other ES clients) have this functionality as independent APIs. Some clients, like python and perl, have a "helper" which wraps the two APIs together so you have to write less code. But they aren't doing anything special.
Reindexing is just taking a document and indexing it into a new index, using basic ES functionality.