ln -s /Applications/MAMP/bin/php/php5.6.10/bin/php /usr/local/bin/php

or

export PATH=/Applications/MAMP/bin/php/php5.6.10/bin:$PATH

Test

php -v

Before:

PHP 5.5.36 (cli) (built: May 29 2016 01:07:06) 
Copyright (c) 1997-2015 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2015 Zend Technologies

After:

PHP 5.6.10 (cli) (built: Jul  6 2015 14:28:54) 
Copyright (c) 1997-2015 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2015 Zend Technologies