Устанавливать будем в свой домашний каталог.
cd ~
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('SHA384', 'composer-setup.php') === 'e115a8dc7871f15d853148a7fbac7da27d6c0030b848d9b3dc09e2a0388afed865e6a3d6b3c0fad45c48e2b5fc1196ae') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
mkdir bin
php composer-setup.php --install-dir=bin --filename=composer
echo $PATH
echo 'export PATH="$PATH:$HOME/bin"' >> ~/.bashrc
source ~/.bashrc
composer