https://phpstan.org/r/51d146e4-ee31-4e29-958d-a8c17c049da7

$urlParts = parse_url($url);
if ($urlParts === false) {
    throw new \InvalidArgumentException('Url is not valid');
}

if (isset($urlParts['host'])) {
    // This is not redundant
    if (isset($urlParts['port'])) {
        echo 'Host and port are defined';
    }
}

Offset 'port' on array('scheme' => string, 'host' => string, 'port' => int, 'user' => string, 'pass' => string, 'path' => string, 'query' => string, 'fragment' => string) in isset() always exists and is not nullable.

I think this is because the return type of parse_url is defined as array()|array(scheme: string, ...)|false which is not correct. The correct type would be array(scheme?: string, ...)|false but I'm not sure if PHPStan is capable of defining optional keys in array literals right now.

This only occurs with bleeding edge. Not sure why.