How to check SSL certificate expiration date in PHP

1<?php
2function daysLeft(string $path): int
3{
4    $info = openssl_x509_parse(file_get_contents($path)); // PEM text
5    if ($info === false) {
6        throw new RuntimeException("$path: not a certificate: " . openssl_error_string());
7    }
8    $notAfter = new DateTimeImmutable('@' . $info['validTo_time_t']); // a Unix timestamp, UTC
9    $left = intdiv($info['validTo_time_t'] - time(), 86400);
10    echo "$path: notAfter {$notAfter->format(DATE_ATOM)}, $left days left\n";
11    return $left;
12}
13 

1<?php
2function serverNotAfter(string $host, int $port, ?string $caFile = null): DateTimeImmutable
3{
4    $ssl = ['capture_peer_cert' => true, 'peer_name' => $host]; // verify_peer is on by default
5    if ($caFile !== null) {
6        $ssl['cafile'] = $caFile; // a private CA instead of the system bundle
7    }
8    $context = stream_context_create(['ssl' => $ssl]);
9    $warnings = []; // a failed handshake reports through warnings, the first one names the cause
10    set_error_handler(function (int $level, string $message) use (&$warnings): bool {
11        $warnings[] = $message;
12        return true;
13    });
14    try {
15        $client = stream_socket_client("ssl://$host:$port", $errno, $error, 10, STREAM_CLIENT_CONNECT, $context);
16    } finally {
17        restore_error_handler();
18    }
19    if ($client === false) {
20        throw new RuntimeException('TLS connection failed: ' . ($warnings[0] ?? $error));
21    }
22    $cert = stream_context_get_params($client)['options']['ssl']['peer_certificate'];
23    fclose($client);
24    return new DateTimeImmutable('@' . openssl_x509_parse($cert)['validTo_time_t']);
25}
26