Updating PHP on MacOS for composer purposes - the requested PHP extension zip is missing from your system

You should have BREW installed and then run

brew install php@7.4

If you get problem with linking newly installed PHP and get error like

Could not symlink sbin/php-fpm
/usr/local/sbin is not writable.

next thing to do is this.
 

The sbin folder is no longer created by default in High Sierra and Mac OSx after it. Here is a nice extract explaining the folders purpose 

/bin
    This directory contains executable programs which are needed
    in single user mode and to bring the system up or repair it.

/sbin
    Like /bin, this directory holds commands needed to boot the 
    system, but which are usually not executed by normal users.

So to solve the problem, simply create the directory with sudo mkdir /usr/local/sbin and then set the correct ownership on it with sudo chown -R `whoami`:admin /usr/local/sbin
dddd
After that check that run (can be done without force modifier also)

brew link php@7.4 --force

and then check with php --version if version is correct.

Another issue you might have now with Composer is that there is not enough memory for it as it is newly installed php and hasn't got enough memory by default which is usually 128MB, so tho check this first run

 php -r "echo ini_get('memory_limit').PHP_EOL;"

and then to get where to change the value, run

php -i | grep php.ini

after that, change the value to some higher number like 2GB or just put -1 for unlimited and try to run composer again.