Do I need a full node to perform OP_RETURN transactions? I want to build a services that allows people to perform OP_RETURN for proof of existence. Do I need to run a full node to do this (then e.g. using the OP_RETURN PHP scripts from https://github.com/coinspark/php-OP_RETURN)? I have found no API that allows to do this.

Thanks
Chris
You do not need to run a full node in order to submit a transaction, regardless of which OP codes that transaction contains.  Any node that can submit transactions to the Bitcoin network will do.

What you should look for instead, is a library in your chosen language that allows you to construct transactions manually, rather than using templates for typical payment transactions.  Once you have a transaction constructed, you can broadcast it to the network via whichever type of node you feel like.