In my mac (version 10.13.6 High Sierra) php -v output is PHP 7.1.32 . 
I need to update this 7.1 version to 7.3.
i tried to remove this version using brew unlink php7.1 but it's not worked.
How can i upgrade php version.
$ brew upgrade php : ( get the latest homebrew php packages )
$ brew install php@7.4 : ( Install php 7.4 )
$ brew link php@7.4 : ( create an alias to this keg-only version; see comments output during installation )
$ echo 'export PATH="/usr/local/opt/php@7.4/bin:$PATH"' >> ~/.bash_profile : ( Add the alias to your path; see comments output during installation )
$ source ~/.bash_profile : ( Reload .bash_profile to use the new settings immediately )
Reference : How to use the php that brew installed?
 
    
     
    
    First:
~ brew update
then install the latest php version (PHP 8.0.8 in the time of writing):
~ brew install php 
link new version to /usr/local/Cellar/php/8.0.8:
~ brew link php
then open the shell's resource file, located in ~/ (in my case Z-shell or ZSH):
~ sudo vi .zshrc
press i to insert and somewhere in your resource file append a new path to your $PATH variable, like this:
export PATH=$PATH:/usr/local/opt/php@8.0/bin
or in the case of ZSH, you should also be able to do it like this:
path+=('/usr/local/opt/php@8.0/bin')
export PATH
then press esc to escape from insert mode, and press :wq to save/write and quit the editor. The final step is to source the file to apply new changes permanently.
~ source .zshrc
Try new PHP version:
~ php -v
 
    
    I am not sure about the 10.13.6 High Sierra version, but this worked for me.
php -v (To see what version php has. But you have already done that)
brew unlink php55 (In my case I use php 5.5)
brew install php73 (For the php 7.3 version.)
php -v (To check of the version is the one I need.)
Hope this helps.
 
    
    I solved the problem maintaining homebrew for both of the arch (arm64 and x86_64).
In that case I choose Homebrew location dynamically depending on the terminal arch in my .bashrc.
if [[ $(uname -m) == "x86_64" ]]; then
  echo "x86_64 detected" 
  export PATH=/usr/local/Homebrew/bin:$PATH
else
  echo "arm64 detected"
  export PATH=/opt/homebrew/bin:$PATH
fi
And installed tmux in both of the terminal (native and rosetta) and after reload it works correctly. Full .zshrc can be found here.
