client.php

<?php

$c = curl_init();

curl_setopt_array($c, [
    CURLOPT_URL => 'http://localhost:8181/index.php',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HEADER => true,
    CURLOPT_COOKIEJAR => 'cookie.txt',
    CURLOPT_COOKIEFILE => 'cookie.txt',
    CURLOPT_HTTPHEADER => [
        'User-Agent: Test-Client',
        'Accept-Encoding: gzip, deflate, br',
        'Accept-Language: en-US,en;q=0.9',
    ],
    CURLOPT_FOLLOWLOCATION => true,
]);

file_put_contents('output.log', curl_exec($c));

curl_close($c);

index.php

<?php

// run the following to start a web server:
// php -S localhost:8181

setcookie('abc123', 'Hello world!');
header('Location: http://localhost:8181/read.php');

read.php

<?php

if (! isset($_COOKIE['abc123'])) {
    http_response_code(400);
    exit('Cookie was not set');
}

echo $_COOKIE['abc123'];