In my experience, you will always feel like you "don't know enough", but that's because there's just too much to know and only a finite amount of time. This shouldn't discourage you, though. I learned way more while actually working than I did in college - and I was hired by my college a couple weeks before graduation.
You would be applying for a junior dev position. In these cases they understand you have no (or very little) practical experience outside of your education. You might even be shadowed or paired up with an intermediate developer to help guide you. Honestly I wouldn't worry about this sort of thing. If you were to apply for a senior or intermediate position, then yeah, you should probably be worried as you probably wouldn't be able to pass the interview. In other words, be truthful, but don't sell yourself short. There's nothing wrong with admitting you don't know something but are willing and perhaps even excited to learn.
On a related note, you won't know everything or remember all the syntax. I can't even begin to count the number of times I've googled simple PHP functions because I forget what the arguments are (until I picked up a decent IDE). Knowing everything doesn't make you a great developer. What's far more valuable is knowing how to find the answer. Be familiar with the PHP Manual (basically php.net), Stack Overflow, and whatever documentation for whatever framework or CMS you may be using. Basically just know where to find an answer. These resources are not online just for junior developers or beginners; they are a valuable resource for everyone who works in PHP or whatever framework/CMS. I do majority of my work in WordPress at my job; I still look up the simplest of functions in the WordPress Codex. But the important thing is I know where to look. The new guys who started here a couple months ago - I'm trying to guide them in that direction. These resources are for all of us, not just the beginners.
My advice to you is to code code code code! Always be coding. You will learn way more by actually doing than anything else. This doesn't mean to skip the research/education part, that's still important - you can't go in blind, but you can only read/Google so much before you need to start writing code. If you are one of those people that don't know what to code, first try something simple at first. It doesn't necessarily have to have any real-world practicality - but it will gain you experience. Then, when you are finished with what you have created - make it again. But this time do something different. Maybe the first iteration was purely procedural code. The second time try doing the same thing but taking an object-oriented approach. Then the third time expand on that and try using an abstract class, or an interface. Even if it's overkill for what you are doing, you are learning the syntax and at least the base knowledge for implementing something. Then maybe try implementing a framework, or using MySQLi instead of PDO, etc. Just small, iterative things to gain you experience.
Start small. You must walk before you run.
Also take advantage of websites like Github or Bitbucket that allow you to host your code and gain feedback (version control is something that will become useful - but one thing at a time). Or simply post code here, or other forums where you can receive feedback.
One last thing before I bring this long post to a close: If you are worried about not knowing enough or making mistakes - just ask. I used to be very quiet and would rarely ask a superior or a coworker for an answer. One day in bit me in the ass and I was nearly fired. From that moment on I just asked. When I moved to my current job I brought that mentality with me and my boss loves me for it. Even if you piss off the person you are talking to because you had to ask the same question three times (though you could always write it down :p), it's better than putting something incorrect into production and having an unhappy client, or something else failing that affects people. People would rather be safe than sorry. And of course as you gain experience you will have to ask less and less, and pretty soon others will be asking you 🙂
