Implementing a full node in any language is extremely difficult because you need to match the behavior of Bitcoin Core exactly, even including bugs and unknown behavior. Don't try it.
May be it is difficult, but not impossible. There is already a full node implementation in Go -
https://github.com/btcsuite/btcd.
It has occurred to me that it might be useful to do payment processing or a wallet by writing a non-verifying "node" in PHP which talks exclusively to a trusted full node over a secure connection (note that Bitcoin network connections are not by themselves secure). I wrote some working PHP code for Bitcoin protocol communication back when I was running blockexplorer.com. It's not that difficult.
The basic requirement to run core on the web is a VPS, which is costly. Not to mention, there are technical complexities as well. I believe, it would be far easier for an average web developer to run a full node, if it were available in PHP-MySQL format and thereby increasing the number of full nodes to secure the bitcoin network. This can be run on a shared hosting if it is used only for validation and providing network data, but not as a wallet.
