Congrats!
First day is usually a right-off due to accounts, environment setup, fire drills, health n safety, bank info etc. So you'll be fine first day. Make sure you're polite to absolutely everyone you meet - not over the top though. In my opinion, the first day/first few weeks is less about the technical aspect and more about how you interact with team members within the company and potentially clients.

From the sounds of it you'll need to keep practising and researching outside of work hours. No reasonable employer will expect perfection in the first day. No reasonable person should ever expect perfection constantly anyway - people make mistakes - it's what we do.

• If you have questions then make sure you ask them. especially if you're unsure about anything. e.g. lunch, flexitime, listening to music on youtube, software downloads. The list can go on.
• Try to figure out who's who and who can solve specific problems etc.
• Try to get a feel for the way people within the company interact with one another so you can understand the relationships.
• Don't complain about anything.
• Can work late within the first couple of weeks so that they understand you care about getting up to speed and helping the company push forward. This is kind of double edged though - if you don't work late then they can't do anything about that but equally it's a good idea, especially given it's your first dev job due to the fact you're trying to build a rapport with the company and its people.
• Have a good handshake.
• Dress sensibly. Most places are hoodies and tshirts but i'd go smart-casual initially. This part isn't essential, but could help with showing people that you can be taken seriously, even if you do make lots of mistakes.
• Whilst at work, research as you go. Waiting for code to compile - then look at some docs etc. No-one should ever be expected to remember the entire documentation for a language.
• Take a note pad and 2 pens - and write down everything.
• Find out what versions of PHP and any frameworks they are using so you can begin to understand the nuances of each and how they compare to each other. (Specific to your situation)

Since it is your first position and I assume entry level, they shouldn't expect too much from your knowledge base yet. All the above should put you in good stead. It's good you've already identified your weaknesses and you're working towards fixing that - constantly improving and learning is a great thing to do! Good job!

Help this helps.