How to verify a certificate chain against your own trust store in PHP

1<?php
2// true: a valid path to a trusted root exists for this purpose; false: it does not; -1: an error
3function verifyChain(string $leafFile, ?string $intermediatesFile, string $rootFile): bool|int
4{
5    return openssl_x509_checkpurpose(
6        file_get_contents($leafFile),
7        X509_PURPOSE_SSL_SERVER,       // also checks the key usage and extended key usage
8        [$rootFile],                   // trusted: files (or directories) with root certificates
9        $intermediatesFile,            // untrusted: a PEM file with intermediates, or null
10    );
11}
12 
13foreach ([
14    'good chain' => ['leaf.pem', 'intermediate.pem', 'root.pem'],
15    'impostor root' => ['leaf.pem', 'intermediate.pem', 'impostor-root.pem'],
16    'expired leaf' => ['expired.pem', 'intermediate.pem', 'root.pem'],
17    'missing intermediate' => ['leaf.pem', null, 'root.pem'],
18] as $label => [$leaf, $intermediates, $root]) {
19    $result = verifyChain($leaf, $intermediates, $root);
20    echo $label, ': ', var_export($result, true);
21    while (($error = openssl_error_string()) !== false) {
22        echo ' | ', $error;
23    }
24    echo "\n";
25}
26 

How it works

1. The third argument lists trusted roots as files or directories (hashed c_rehash layout); only those are trusted.
2. The fourth argument is a file of untrusted intermediates — one PEM file that may hold several certificates.
3. The purpose adds key usage checks: X509_PURPOSE_SSL_SERVER requires a certificate fit for TLS servers, X509_PURPOSE_ANY skips that part.
4. true means OpenSSL found and verified a path: signatures, validity, CA flags — the same engine as openssl verify.

Gotchas

• On failure the function returns false and gives no reason: the run's look-alike root, expired leaf and missing intermediate all print false with no message in openssl_error_string(). Run openssl verify on the same files to see why.
• A CA file that does not exist also yields false — indistinguishable from an untrusted chain — so check that your trust files are readable before blaming the certificate.
• The function can also return -1 for internal errors; compare with === true, never with a plain truthiness check, because -1 is truthy.
• Roots and intermediates are file paths, not PEM strings; write PEM data you hold in memory to temporary files first.

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