To generate an ECDSA key pair in PHP, call openssl_pkey_new with OPENSSL_KEYTYPE_EC and a curve name, then export both halves: openssl_pkey_export writes the PKCS#8 private key, openssl_pkey_get_details returns the SPKI public key PEM. Everything is in the OpenSSL extension that ships with PHP — the only trap is the curve name.

Tested with PHP 8.5.11 (ext/openssl, OpenSSL 3.5.7) and OpenSSL 3.5.7 (openssl pkey cross-check).

generate.php — a P-256 key pair written as PEM files

1<?php
2$key = openssl_pkey_new([
3    'private_key_type' => OPENSSL_KEYTYPE_EC,
4    'curve_name' => 'prime256v1', // P-256 under its OpenSSL name; 'P-256' is not accepted
5]);
6if ($key === false) {
7    throw new RuntimeException('key generation failed: ' . openssl_error_string());
8}
9 
10openssl_pkey_export($key, $privatePem); // "BEGIN PRIVATE KEY" (PKCS#8)
11$publicPem = openssl_pkey_get_details($key)['key']; // "BEGIN PUBLIC KEY" (SPKI)
12 
13$umask = umask(0077); // the private key file is created owner-only
14file_put_contents('key.pem', $privatePem);
15umask($umask);
16file_put_contents('pub.pem', $publicPem);
17echo $publicPem;
18

encrypted.php — a passphrase-protected private key

1<?php
2// "BEGIN ENCRYPTED PRIVATE KEY": the same export, with a passphrase and a cipher
3$key = openssl_pkey_new(['private_key_type' => OPENSSL_KEYTYPE_EC, 'curve_name' => 'prime256v1']);
4openssl_pkey_export($key, $encryptedPem, getenv('KEY_PASSPHRASE'), [
5    'encrypt_key_cipher' => OPENSSL_CIPHER_AES_256_CBC,
6]);
7file_put_contents('key-encrypted.pem', $encryptedPem);
8file_put_contents('pub-encrypted.pem', openssl_pkey_get_details($key)['key']);
9

How it works

openssl_pkey_new takes the key type and curve_name; openssl_get_curve_names() lists every name the linked OpenSSL accepts.
openssl_pkey_export writes PEM into its second argument; with OpenSSL 3 that is PKCS#8, BEGIN PRIVATE KEY.
openssl_pkey_get_details($key)['key'] is the public key as SPKI PEM; the same array holds the curve name and the coordinates under ec.
umask(0077) around file_put_contents creates key.pem with mode 600 — the run checks the mode and that OpenSSL derives the same public key from it.
With a passphrase and encrypt_key_cipher, the export becomes BEGIN ENCRYPTED PRIVATE KEY, which openssl_pkey_get_private($pem, $passphrase) and openssl pkey -passin both open.

Gotchas

Only OpenSSL short names work: 'prime256v1' for P-256, 'secp384r1', 'secp521r1', 'secp256k1'. 'P-256' and even 'secp256r1' make openssl_pkey_new return false.
After a successful openssl_pkey_new the OpenSSL error queue may still hold random number generator::Cannot open file (OpenSSL looks for a ~/.rnd seed file). It is harmless, but code that treats any openssl_error_string() as failure misfires — check the return value instead.
openssl_pkey_get_details on a private key also returns the secret scalar as ['ec']['d']. Take only ['key'] from it, and never log or serialize the whole array.
umask is process-wide; the PHP manual advises against it in multithreaded web servers. There, create the file with fopen, chmod it to 0600, and only then write the key.

How to generate an ECDSA key pair in PHP

How it works

Gotchas

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