I work with WordPress on a daily basis and I am very experienced with it (it gets a lot of hate but I think it's just misunderstood 🙂), so if you have questions feel free to ask.
To be honest I am not quite sure what you're trying to achieve, but based on your second post here it sounds like you're dealing with a shortcode. Personally I avoid them whenever possible but they are great if they're used correctly. Let me ask you this question: Is this PHP code that you're trying to add in going to be on all pages, just this specific page, etc.? A simple way of solving this (without a shortcode) is to create a template page which is usually based on the general content page. From here you can do whatever you want with it from a developer's standpoint. Then when you create the page in the WordPress back-end you choose the template from the drop down menu on the right side to the one you just created. In some cases the template pages are nearly identical to regular pages but will have some special code running in them (that's how I usually do things).
Even with a shortcode, you still have to write out what it does, so when WordPress encounters it, it knows what to do. One instance where you would need to use the shortcode is if the client needs the ability to change/add/remove the functionality that you're trying to add right now. Simply put, shortcodes allow the client to add PHP code within the content editor.
If you have more questions please ask =]