やったこと

php7.3のインストール
環境変数の設定
echo 'export PATH="/usr/local/opt/php@7.2/bin:$PATH"' >> /Users/xxx/.bash_profile
php -v で確認

起きたこと

dyld: Library not loaded: /usr/local/opt/openssl/lib/libcrypto.1.0.0.dylib
  Referenced from: /usr/local/bin/php
  Reason: image not found

解決

source ~/.bash_profile

↑これで設定が再読み込みされる

PHP 7.3.21 (cli) (built: Aug  7 2020 18:56:36) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.21, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.3.21, Copyright (c) 1999-2018, by Zend Technologies

以上