Install Composer

cd /tmp

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"

EXPECTED_HASH=$(curl -s https://composer.github.io/installer.sig)

ACTUAL_HASH=$(php -r "echo hash_file('sha384', 'composer-setup.php');")

if [ "$EXPECTED_HASH" = "$ACTUAL_HASH" ]; then
    echo "Installer verified"
else
    echo "Installer corrupt"
    rm composer-setup.php
    exit 1
fi

php composer-setup.php --version=2.3.10 --install-dir=/usr/local/bin --filename=composer74

rm composer-setup.php

Even simpler (if you trust HTTPS)

If you're OK skipping hash validation:

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php composer-setup.php --version=2.3.10 --install-dir=/usr/local/bin --filename=composer74
rm composer-setup.php

For most servers behind HTTPS, this is acceptable.