Enable PHP extension

Since I use homebrew to install PHP (Command: brew install php56)

And so, we'll have my php.ini file located in other directory.
which is,

/usr/local/etc/php/5.6/php.ini

so you'll have to edit this file in order to activate extension.
In the file, in the Dynamic Extensions part, you will see a list of commented out extensions.

;extension=php_bz2.dll
;extension=php_curl.dll
;extension=php_fileinfo.dll
;extension=php_gd2.dll
;extension=php_gettext.dll
;extension=php_gmp.dll
;extension=php_intl.dll
;extension=php_imap.dll
;extension=php_interbase.dll
;extension=php_ldap.dll
;extension=php_mbstring.dll

Install desired extension eg.

brew install php56-mcrypt

remove the semicolon ";" at the desired extension to enable it.
and then restart apache server with

sudo apachectl restart 

your extension would be ready for action.