Pruning is helpful if you want to use bitcoind.
As for a lightweight client in PHP - there isn't really such a project. You'll find lots of clients which expose an RPC port, allowing you to use any language to talk to them.
You can also talk to stratum servers. Electrum clients do this. It's allows for SPV level security about payments. There's actually a list of 20 or so public servers, you just need to write a library to talk to them.
The messages are JSON, with new lines indicating the end.
I've written a hacky library to talk to electrum servers in PHP, but since it's async it might be more pain than it's worth.
https://github.com/Bit-Wasp/stratum-php