That's because it's referring to the pre-installed PHP of your OSX. You need to change that to point to your MAMP PHP. To do so you need to edit the .bash_profile with a text editor and add the MAMP version of PHP to the PATH variable.
Follow these steps (source):
Within the Terminal, run
vim ~/.bash_profile
Type i and then paste the following at the top of the file:
export PATH=/Applications/MAMP/bin/php/php7.1.1/bin:$PATH
Hit ESC, Type :wq, and hit Enter
In Terminal, run:
source ~/.bash_profile
In Terminal, type in which php again and look for the updated string. If everything was successful, It should output the new path to MAMP PHP install.
In case it doesn't output the correct path, try closing the terminal window (exit fully) and open again, it should apply the changes.