The issue is related with the openssl package.
I found the solution here:
https://stackoverflow.com/questions/68957915/macos-10-12-brew-install-openssl-issue
Edit the openssl with the following `brew edit openssl` and add the line:
``` -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include ```
To args array in `configure_args`, as below:
``` def configure_args args = %W[ --prefix=#{prefix} --openssldir=#{openssldir} no-ssl3 no-ssl3-method no-zlib ##### add the line here #### -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include ] on_linux do args += (ENV.cflags || "").split args += (ENV.cppflags || "").split args += (ENV.ldflags || "").split args << "enable-md2" end args end ```
Then you try again to install the PHP. I installed php@7.4.
``` brew install php@7.4 ```

I managed to resolve it by editing formula (`brew edit openssl`) and adding
``` -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include ```
to args array in `configure_args`.

As below:
``` def configure_args args = %W[ --prefix=#{prefix} --openssldir=#{openssldir} no-ssl3 no-ssl3-method no-zlib ##### add the line here #### -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include ] on_linux do args += (ENV.cflags || "").split args += (ENV.cppflags || "").split args += (ENV.ldflags || "").split args << "enable-md2" end args end ```