Add a PSR-4 namespace to a composer.json file

psr-4-add.php

#!/usr/bin/env  php
<?php
file_put_contents(
    'composer.json',
    json_encode(array_merge_recursive(
        ['autoload' => ['psr-4' => [$argv[1] => $argv[2]]]],
        json_decode(file_get_contents('composer.json'), true)
    ))
);

<?php
file_put_contents(
    'composer.json',
    json_encode(array_merge_recursive(
        ['autoload' => ['psr-4' => [$argv[1] => $argv[2]]]],
        json_decode(file_get_contents('composer.json'), true)
    ), JSON_PRETTY_PRINT)
);

Life is too short to look up the PHP docs, looking for non-class constants