Update

For anyone that just want xdebug support on MacOS, most of the instructions in this answer are not necessary when using the built-in version of PHP. Before doing anything, you should check if xdebug.so already exists in /usr/lib/php/extensions/no-debug-non-zts-20180731/, which should be there by default. If so, you can skip to the Enabled support in PHP portion of this answer.

Using Homebrew is also an acceptable solution for you (and can also prevent other issues).

For anyone else looking to actually build binaries on MacOS and get the header error, the full answer is for you. It also answer OP question directly. Note building xdebug from source code and actually trying to use that version of xdebug.so with the build-in PHP should end up in a "code signature" error. As described here and here, the only real solution would be to compile and use you own instance of PHP instead of the built-in one. In any situation, using Homebrew would be easier.

tl;dr

Apple decided to remove headers file in /usr/include and the macOS_SDK_headers_for_macOS_10.14.pkg package. To install Xdebug, you'll have to manually compile Xdebug with the correct reference in both phpize and make.

For more details, I wrote a blog article about the issue and the solution

Original Answer:

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.

First, make sure Xcode is installed, including the command line tools. The following command will display the location of the default SDK :

$ xcrun --show-sdk-path/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk

The header you'll want (php.h) will then be in /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php/main.

Getting source

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

git clone https://github.com/xdebug/xdebug.gitcd xdebuggit checkout tags/2.7.2

phpize

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

cp /usr/bin/phpize .nano ./phpize

Find this line :

includedir="`eval echo ${prefix}/include`/php"

...and replace it with this line :

includedir="/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php"

Run phpize:

./phpize

You should now see something like this :

Configuring for:PHP Api Version:         20180731Zend Module Api No:      20180731Zend Extension Api No:   320180731

Configure & build

We can now configure :

./configure --enable-xdebug

...and run make using our custom SDK location defined as compiler flags :

make CPPFLAGS='-I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php/main -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php/TSRM -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php/Zend -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php/ext -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/php/ext/date/lib'

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

make install

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

Next, we move the executable somewhere safe. I use /usr/local/php/extensions.

sudo mkdir -p /usr/local/php/extensionssudo cp /usr/lib/php/extensions/no-debug-non-zts-20180731/xdebug.so /usr/local/php/extensions

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

sudo nano /etc/php.ini

And we add the following at the bottom :

[xdebug]zend_extension=/usr/local/php/extensions/xdebug.soxdebug.remote_enable=onxdebug.remote_log="/var/log/xdebug.log"xdebug.remote_host=localhostxdebug.remote_handler=dbgpxdebug.remote_port=9000

Restart built in server to be sure :

sudo apachectl restart

And finally test everything went fine :

php -i | grep "xdebug support"

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

Note:A more complete fix would be to edit the result of php-config --include-dir, which returns /usr/include/php. That would make any installation find the necessary header files without having to manually edit files or compiler flags.

If you are using brew, I solve this by reinstalling php and re-linking:

brew reinstall php@7.3brew link --overwrite php

I got an error when I tried to install xdebug in MacOS Catalina 10.15:

pecl install xdebug-3.0.1

Error:

/private/tmp/pear/install/xdebug/xdebug.c:25:10: fatal error: 'php.h' file not found

This is due to Apple decided to remove headers file in /usr/include, like you can see in other answer.

Then I added config to .bash_profile, executing these lines in console:

echo 'export PATH="/usr/local/opt/php@7.3/bin:$PATH"' >> ~/.bash_profileecho 'export PATH="/usr/local/opt/php@7.3/sbin:$PATH"' >> ~/.bash_profileexport LDFLAGS="-L/usr/local/opt/php@7.3/lib"export CPPFLAGS="-I/usr/local/opt/php@7.3/include"source ~/.bash_profile

After that you can try to install xdebug again with pecl:

pecl install xdebug-3.0.1

Notes: Previously I have installed PHP 7.3 with "brew". You should adjust the php and xdebug version, in the above lines, adding the versions that you prefer.