As Waqleh said, you have to uninstall PHP Composer and install it again. First, execute:
sudo apt-get remove composer
Then, execute these commands. The checksum here is for Composer 1.10.13, but you'll get the newest Composer (2.0.4 at the moment of editing this answer) when running the first line, so be sure to check in https://getcomposer.org/download/:
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '8a6138e2a05a8c28539c9f0fb361159823655d7ad2deecb371b04a83966c61223adc522b0189079e3e9e277cd72b8897') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php
php -r "unlink('composer-setup.php');"
Now move file composer.phar to a directory that is in your path (from Installation - Linux / Unix / macOS):
sudo mv composer.phar /usr/local/bin/composer
And execute composer
from any directory. That's all!
PS: If you're using PhpStorm (or maybe other IDEs), you'll have to close it and open it again.
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