With vanilla PHP it's possible to check of if the SSL certificate of a given site is valid. But it's kinda madness to do it. Let's look at the code required:

<br />// Step 1: downloading the certificate from the site
$streamContext = stream_context_create([
    'ssl' => [
        'capture_peer_cert' => true,
    ],
]);

$client = stream_socket_client(
    "ssl://spatie.be:443",
    $errorNumber,
    $errorDescription,
    $timeout,
    STREAM_CLIENT_CONNECT,
    $streamContext);

$response = stream_context_get_params($client);

$certificateProperties = openssl_x509_parse($response['options']['ssl']['peer_certificate']);

// Step 2: parsing the certificate

/*
* I'm not even going to type out the further code needed.
*
* `$certificateProperties` has two keys `validFrom_time_t` and `validTo_time_t`. 
* Those keys contain the UTC representation of the date.
* You will need to check if the current date is between those dates.
*/ 

What. The. Actual. F. Let's fix this!

We've released a new package named spatie/ssl-certificate that makes checking the SSL certificate of a site laughably easy. Let's take a look at the code:

$certificate = SslCertificate::createForHostName('spatie.be');
$certificate->isValid(); // returns true if the certificate is currently valid

Boom, done.

The package has a few more methods that makes working with an SSL certificate a breeze:

$certificate->getIssuer(); // returns "Let's Encrypt Authority X3"

$certificate->getDomain(); // returns "spatie.be"

//A certificate can cover multiple (sub)domains. Here's how to get them.
$certificate->getAdditionalDomains(); // returns ["spatie.be", "www.spatie.be]

$this->certificate->validFromDate(); // returns an instance of Carbon

$certificate->getExpirationDate(); // returns an instance of Carbon

You can also use isValid to determine if a given domain is covered by the certificate. Of course it'll keep checking if the current datetime is between validFromDate and expirationDate.

$this->certificate->isValid('spatie.be'); // returns true
$this->certificate->isValid('laravel.com'); // returns false

The source code of the package is available on GitHub. My company has made many more PHP framework agnostic, Laravel and JavaScript packages in the past. Take a look at the open source page at our site to see if we've made anything that could be of use to you.