Yes, PHP errors are shown at runtime, even when they're just syntax errors that could have been found out while compiling. That's because no real compilation happens; RapidEXE works by giving your sources a separated little environment where they can run and it has the whole PHP (or Python) compiler built in. So the actual "compilation", which is really just interpretation by php, happens when you run the program.

It's probably related to PHP extensions. Check the php.ini for the engine you're using; RapidEXE has a folder called engines and within that, choose the one your project runs with. If you never changed it, php.7.2.win64 is the default one. This is where you'll have to add some DLLs... See the next point:

Adding DLLs are easy, but there are some non-trivial steps to take. First of all, you need the right DLL for the PHP version you're using. If it's the same version you use on your machine, just copy it; if not, download the right version from the official PHP site.

Note: if you downloaded something and it's not compatible with the engine, a quick fix is to replace the other php files in the engine folder with those you just downloaded.

Once the DLL is there, you need to perform 2 more steps: update php.ini and empty RapidEXE cache. Step 1 is easy - for example, if mb_string is the extension you wanted, just uncomment this line:

 extension=./php_mbstring.dll 

Notice the relative path. This is important; extensions will not have this by default. Add it to ensure PHP will look for the extension in its own folder (where php.exe resides), otherwise it will use the path of your default php environment. Once you're done here, you need to force RapidEXE to recompile the engine so you want to empty its cache. Press Win+R to open the "Run" window and enter this path:

 %temp%\rapidexeTemp 

Click OK! The RapidEXE temp folder will open, with 3 subfolders in it:

 compiler
 engine
 script

And all you have to do is delete them. All 3 of them. Technically compiler and engine should be enough but it's easier to just say Ctrl+A, Del, Yesplease.

Now the next time you compile your project, RapidEXE will pack the engine again (instead of using the cached version) so it will take a second longer or two; but then, everything should work and the next time you compile, it will be fast again (since now the modified version of the engine will be in the cache).