I'm not familiar with this library, nor with the electrum json-rpc api (i always use the core implementation's json-rpc api), but judging from this page:
http://docs.electrum.org/en/latest/merchant.html#create-a-signed-payment-request

I suppose it has to be something like this:
$response = $electrum->addrequest(1, "test");

This is completely untested (i don't have an electrum daemon running, nor an nginx stack, nor a rPi,... So i can only point towards what i think might be a sollution... It's up to you to test and debug).