The developers who were keeping the official neo4j php client updated are currently focused on other things. There are still a large number of open-source, up-to-date php clients out there. Additionally, If you take a closer look at some of those clients, and how they interface with the bolt protocol, I think you'll quickly find the reason why a php client isn't really a major concern: the bolt protocol already does what a standard php mysql connection does.
Do you really need a mysql-client to use mysql in php? No. However, sometimes the convenience of pre-built structures and patterns to handle the request/response lifecycle do make development a bit easier and faster. However, any such client will bring with it some rigidity and patterns that you will be forced to incorporate.
Graphaware's php-neo4j-client has not yet been updated to connect to Neo 4, but someone is going to pick it up. Even if they don't, remember that all these clients are doing are the exception handling for connecting to bolt, sending Cypher queries, and cleaning up the results.
This looks like a fairly active fork, that has managed some 4.0 stability, and isn't far from 4.1:
If you really want the crutch ;) , I'm sure there's others out there.