How to create a self-signed EC P-256 certificate with SANs in PHP

1# cert.cnf — openssl_csr_sign takes certificate extensions only from a config section
2[ req ]
3distinguished_name = req_dn
4 
5[ req_dn ]
6 
7[ server_cert ]
8subjectAltName   = DNS:localhost, IP:127.0.0.1
9basicConstraints = critical, CA:FALSE
10keyUsage         = critical, digitalSignature
11extendedKeyUsage = serverAuth

1<?php
2$options = ['config' => __DIR__ . '/cert.cnf', 'x509_extensions' => 'server_cert', 'digest_alg' => 'sha256'];
3 
4$key = openssl_pkey_new(['private_key_type' => OPENSSL_KEYTYPE_EC, 'curve_name' => 'prime256v1']); // P-256
5$csr = openssl_csr_new(['commonName' => 'localhost'], $key, $options);
6 
7// CA certificate null = self-signed; validity in days, counted from now
8$cert = openssl_csr_sign($csr, null, $key, 365, $options, random_int(1, PHP_INT_MAX));
9if ($cert === false) {
10    throw new RuntimeException(openssl_error_string());
11}
12 
13openssl_x509_export($cert, $certPem);
14openssl_pkey_export($key, $keyPem);
15file_put_contents('cert.pem', $certPem);
16file_put_contents('key.pem', $keyPem);

How it works

1. openssl_pkey_new with OPENSSL_KEYTYPE_EC and curve_name => 'prime256v1' creates a P-256 key; openssl_csr_new builds a CSR for commonName => 'localhost'.
2. openssl_csr_sign($csr, null, $key, 365, $options, $serial) signs the request with its own key — null as the CA certificate means self-signed — for 365 days from now.
3. x509_extensions => 'server_cert' names the cert.cnf section with subjectAltName, basicConstraints, keyUsage and extendedKeyUsage; digest_alg => 'sha256' sets the signature hash.
4. openssl_x509_export and openssl_pkey_export return the PEM strings for cert.pem and key.pem.

Gotchas

• The serial number defaults to 0 (serial=00 in OpenSSL's output), which RFC 5280 does not allow — serials must be positive. Pass a random one, such as random_int(1, PHP_INT_MAX).
• Without digest_alg, the certificate is signed with SHA-1 (ecdsa-with-SHA1); pass 'digest_alg' => 'sha256'.
• Without x509_extensions, and with a config whose [ req ] section names no default, the certificate has no extensions at all — no SANs and no basicConstraints.
• A misspelt section name does not throw: openssl_csr_sign returns false with only a warning (Error loading extensions_section section missing_section of c.cnf), so check the result before exporting.

Related recipes

• How to generate a CSR for an EC P-256 key with SANs in PHP
• How to create a self-signed EC P-256 certificate with SANs with OpenSSL
• How to create a self-signed EC P-256 certificate with SANs in Ruby
• How to convert a PEM public key to a JWK and back in PHP