test_proxy.php

<?php

$fileUrl = 'http://repo.packagist.org/packages.json';

$options = array (
  'http' => 
  array (
    'follow_location' => 0,
    'max_redirects' => 20,
    'proxy' => 'tcp://127.0.0.1:8888',
    'request_fulluri' => true,
    'protocol_version' => 1.1,
    'header' => 
    array (
      0 => 'Accept-Encoding: gzip',
      1 => 'Connection: close',
      2 => 'User-Agent: Composer/source (Windows NT; 10.0; PHP 7.2.12)',
    ),
    'ignore_errors' => true,
  )
);

$context = stream_context_create($options);

$result = file_get_contents($fileUrl, false, $context);

echo zlib_decode($result);