Is there any standalone script to generate SegWit Address Key pair using PHP?

[CounterEntropy]

Something similar is available for Bitcoin legacy addresses here -
https://github.com/BitcoinPHP/BitcoinECDSA.php/blob/master/src/BitcoinPHP/BitcoinECDSA/BitcoinECDSA.php

[bitmover]

I know there is something for javascript here:

https://github.com/bitcoinjs

Specific discussion for segwit address generation::
https://github.com/bitcoinjs/bitcoinjs-lib/issues/1137

I have also found this Bitcoin PHP library:
https://github.com/Bit-Wasp/bitcoin-php

There is a discussion here about segwit addresses:
https://github.com/Bit-Wasp/bitcoin-php/issues/613

This might help you

[HCP]

As bitmover has found, Bit-Wasp is probably your best bet for PHP specific code...

There are several examples that demonstrate how to generate the p2wpkh (bech32) and p2sh.p2wpkh (nested segwit) addresses listed here: https://github.com/Bit-Wasp/bitcoin-php/blob/1.0/examples/addresstypes.script.php

and other examples of generating keys/addresses here: https://github.com/Bit-Wasp/bitcoin-php/tree/1.0/examples/doc/keys

Indeed, there are lots of fairly useful examples for various use-cases: https://github.com/Bit-Wasp/bitcoin-php/tree/1.0/examples