While trying to compile PHP extensions after installing macOS Mojave, I was butting up against a ton of missing header file errors including php.h and zlib.

After several hours of beating my head against my desk and Googling/Stack Overflowing I found the solution.

Ends up you just need to force re-install the header files. Special thanks to @sfdye for the solution!

$ sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target /

If you get an error about that file not existing, you probably simply don't have the Xcode command line tools installed which can be remedied simply as follows:

$ xcode-select --install

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