After figuring out a way around the massive breaking change introduced by Xcode 11, it's now time to install Xdebug back on MacOS Catalina.

Update from January 6th 2021:
Using MacOS Big Sur? This solution should work for you too. But since Apple Deprecated PHP in MacOS Big Sur, you should probably rely on Homebrew now.

Xcode up to 12.4 will run on Catalina. Xcode 12.5 (when released any day now) and higher will require Big Sur. This is in the release notes for 12.5. I installed xcode long before my Catalina upgrade. I currently don't need the bloat on my mac and was going to try to remove it. However, I'm getting 'You don't have permission to perform this task' when I try to move it to the trash. I am an administrator locally. Appreciate any help.

Update from June 30th 2020:
After some new investigation while trying to upgrade to a newer version of xdebug, I now believe most of the instructions in this post are not necessary. Before doing anything, you should check if xdebug.so already exists in /usr/lib/php/extensions/no-debug-non-zts-20180731/, which I believe is there by default (let me know if otherwise). If it does exist, you could skip to the Enabled support in PHP portion of this post.

1. Yes, you can run Xcode 12 with the current macOS, Catalina. It will work perfectly, although you can update to the developer beta of Big Sur. Yes, it supports Catalina. I recommend using the latest Big Sur beta, however, so that you can update your app to support the latest technologies.
2. For Catalina + Xcode 12 + iOS14 SDK no options for now to build Qt Application it's not supported by Qt 5.15.1. Awaiting announced Qt 6.0 that is going to be supported XCode 12 + iOS14 SDK. For Qt will be working only XCode 11 downloaded manually (do not install from AppStore).
3. Xcode 12.5.1 (Big Sur) has an error compiling cocosBuilder, but I can't find a reason. Other computers (Xcode 12.4 / Catalina) say no errors occur.

Note that building xdebug from source code and actually trying to use that version of xdebug.so (for example by referencing the built file in xdebug/module/xdebug.so after using make install) with the build-in PHP should end up in a 'code signature' error. As described here and here, even after signing the binary, MacOS won't allow system binaries to interact with non-system binaries for security reasons. The only real solution to use a custom version of xdebug would be to compile and use you own instance of PHP instead of the build in one.

Xcode 12 On Catalina

Long story short, Apple decided to nuke /usr/include in MacOS Catalina, which has been the default location for C header file for ever in UNIX systems. Trying to install through PEAR / PECL will return an error as the compiler will look for necessary headers file in /usr/include. So the solution is to compile Xdebug manually, manually specifying the actual location of the header files, which are still provided by Xcode, just at a different location.

Xcode

The first step is to get Xcode from the App Store.

Once Xcode installed, we have to get the command line tools :

You might need to actually open Xcode at this point to finish installation and accept terms and conditions, especially if the previous command fails.

Finally, make sure the SDK is found. If the path you get differs from the one bellow, you might need to edit the path accordingly later on:

Xcode 10 On Catalina

Manually Compiling Xdebug

Getting source

Let's compile 2.7.2, getting the source code from git. Alternatively, you can download the source from Xdebug site.

phpize

Next we need to make a copy phpize so we can edit the include path :

Find this line :

...and replace it with this line :

Xcode 12 On Catalina

Run phpize:

You should now see something like this :

Configure & build

We can now configure :

...and run make using our custom SDK location defined as compiler flags. I used a variable to store the path to the SDK so it's easier to edit if it changes :

You might see some warning, just ignore it for now. Finally, we'll need to run :

Again, this command will fail because it can't move the extension to the right place. SIP will prevent it. But no worries, we'll take care of that manually at the next step. make install is still required as it will sign the *.so file.

Enabled support in PHP

Once make install has been run, we can move the executable somewhere safe. I use /usr/local/php/extensions.

Then we edit the PHP configuration to enable Xdebug. Simply edit php.ini:

Xcode 8 On Catalina

And we add the following at the bottom :

Restart built in server to be sure :

And finally test everything went fine :

If the above command returns nothing, then Xdebug is not available on your install. Go back the steps to find out what's missing.

Update Xcode On Catalina

References and thanks :