PHP generates a CSR with ext-openssl: openssl_pkey_new creates the P-256 key and openssl_csr_new builds and signs the request. Extensions such as subjectAltName can only come from an OpenSSL config section, so the recipe ships a three-line csr.cnf. The run decodes the CSR with OpenSSL.

csr.cnf — the extension section openssl_csr_new reads

1# csr.cnf — openssl_csr_new takes extensions only from a config section
2[ req ]
3distinguished_name = req_dn
4req_extensions     = v3_req
5 
6[ req_dn ]
7 
8[ v3_req ]
9subjectAltName = DNS:api.example.com, DNS:www.example.com

csr.php — P-256 key and CSR with two SANs

1<?php
2$key = openssl_pkey_new(['private_key_type' => OPENSSL_KEYTYPE_EC, 'curve_name' => 'prime256v1']); // P-256
3 
4$csr = openssl_csr_new(
5    ['countryName' => 'US', 'organizationName' => 'Example Corp', 'commonName' => 'api.example.com'],
6    $key,
7    ['config' => __DIR__ . '/csr.cnf', 'req_extensions' => 'v3_req', 'digest_alg' => 'sha256'],
8);
9if ($csr === false) {
10    throw new RuntimeException(openssl_error_string());
11}
12 
13openssl_csr_export($csr, $csrPem);
14openssl_pkey_export($key, $keyPem); // PKCS#8; pass a passphrase to encrypt it
15file_put_contents('csr.pem', $csrPem);
16file_put_contents('key.pem', $keyPem);

How it works

openssl_pkey_new with OPENSSL_KEYTYPE_EC and curve_name => 'prime256v1' creates the key (prime256v1 is OpenSSL's name for P-256).
The first argument of openssl_csr_new is the subject; keys are OpenSSL's long names (countryName, organizationName, commonName).
config points at csr.cnf and req_extensions names its v3_req section, which holds subjectAltName.
openssl_csr_export and openssl_pkey_export return the PEM strings; openssl_pkey_export writes PKCS#8.

Gotchas

Without digest_alg, the request is signed with SHA-1 (ecdsa-with-SHA1 in OpenSSL's output). CAs reject such CSRs; always pass 'digest_alg' => 'sha256'.
There is no option for SANs in the PHP API: without a config section, the CSR has only a CN, which modern clients do not accept for host names.
openssl_csr_new returns false on failure without throwing; check the result and read openssl_error_string() for the reason.
For dynamic SAN lists, either write the config file at runtime or put subjectAltName = $ENV::SAN in it and set SAN=DNS:a.example.com,DNS:b.example.com in the environment — OpenSSL expands it when it reads the section.

How to generate a CSR for an EC P-256 key with SANs in PHP

How it works

Gotchas

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