etherscan-api | etherscan.io API PHP wrapper | REST library

Etherscan doesn't provide pending transactions in their API.

It's probably because of the nature of how pending transactions are collected and displayed. All pending transactions are collected together in the Ethereum mempool. My guess is that Etherscan adds each entry from mempool to some kind of quickly searchable database such as ElasticSearch, and removes the DB entry when the transaction is taken off the mempool. The address detail page then performs a simple search in ES, filtering only transactions from/to this particular address.

If your web3 provider allows websocket connections and subscriptions, you can subscribe to the pendingTransactions event.

It fires an event everytime a new transaction is added to the mempool. The event contains only the transaction hash (and no additional data such as the sender and receiver).

Because the event contains only tx hash, it's not very effecient to always perform a new query on the tx details, but it's useful in some other cases such as when you know the tx hash beforehand and you only want to know when it has been successfully broadcasted.

There's also BlockCypher REST API that has a limited free plan (and somewhat less limited paid plans) that returns all pending transactions including the tx details so that you can perform your own search on the result. See their docs if this fits your use case.