BEHOLD probably one of the hackiest things I've done in a long time

wwwphpcs.php

<?php

require_once( 'vendor/squizlabs/php_codesniffer/autoload.php' );

// set the current working directory as the current directory
chdir( __DIR__ );
$_SERVER['argv'] = [
  'phpcs',
  'test.php',
  '--report=json',
];
ob_start();
$runner = new \PHP_CodeSniffer\Runner();
$exitCode = $runner->runPHPCS();
$phpcs_output = ob_get_clean();

echo "This is the output:\n";
echo $phpcs_output."\n";
echo "all good?\n";

You can also do this without trying to replicate argv. See this example, which uses a dummy file to process content but could be changed to process a file path as well: https://gist.github.com/gsherwood/aafd2c16631a8a872f0c4a23916962ac