Open your CMD Line - Command Prompt
1. Go to your php.exe file by command

cd c:\xampp\php\

2. Type composer instalation command

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('SHA384', 'composer-setup.php') === '55d6ead61b29c7bdee5cccfb50076874187bd9f21f65d8991d46ec5cc90518f447387fb9f76ebae1fbbacf329e583e30') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php
php -r "unlink('composer-setup.php');"

And enjoy result like this

c:\xampp\php>php -r "copy('https://getcomposer.org/installer', 'composer-setup.p
hp');"

c:\xampp\php>php -r "if (hash_file('SHA384', 'composer-setup.php') === '55d6ead6
1b29c7bdee5cccfb50076874187bd9f21f65d8991d46ec5cc90518f447387fb9f76ebae1fbbacf32
9e583e30') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlin
k('composer-setup.php'); } echo PHP_EOL;"
Installer verified

c:\xampp\php>php composer-setup.php
All settings correct for using Composer
Downloading...

Composer (version 1.3.2) successfully installed to: C:\xampp\php\composer.phar
Use it: php composer.phar