openssl_pkey_get_public

If you are trying to read a PKCS#1 RSA public key you run into trouble, because openssl wants the public key in X.509 style.
The PKCS#1 RSA public key
-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEAgYxTW5Yj+5QiQtlPMnS9kqQ/HVp+T2KtmvShe68cm8luR7Dampmb
[...]
cbn6n2FsV91BlEnrAKq65PGJxcwcH5+aJwIDAQAB
-----END RSA PUBLIC KEY-----
.. is not readable while the X.509 style public key
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAgYxTW5Yj+5QiQtlPMnS9
[..]
JwIDAQAB
-----END PUBLIC KEY-----
is. You can use an easy (and dirty) work around to read the PKCS#1 RSA anyway. The first few bytes of the X.509 style public key contain header information and can shamelessly be copied.
In other words: Delete everything after the first 32 bytes from the above X.509 key (starting behind Q8A) and attach your PKCS#1 data, reformat to 64 bytes length and use it with openssl.
Please note: The above example only works for 2048 bit length.
Like I said - it's kind of dirty - but hey - if you're as desperate as I was.
Michaela

I spent a few hours raging with this function and hitting my head on the desk trying to get it to load a public PEM key.
This function can leave errors in openssl_error_string even if it succeeded so this can cause a lot of confusion further down. Especially if you're prototyping and haven't put full checks on return values in yet. The error will not be cleared either when calling other functions successfully.
To avoid confusion, you should always check the return result and only call openssl_error_string after calling an openssl function that returned failure (false).

you can get (and save to file) public key using openssl_pkey_get_details(resource $key ) function:
<?php
$pub_key = openssl_pkey_get_public(file_get_contents('./cert.crt'));
$keyData = openssl_pkey_get_details($pub_key);
file_put_contents('./key.pub', $keyData['key']);
?>

This documentation notes it can take a PEM-formatted private key, but as per bug #25614, this is not possible in any form. The function simply returns a FALSE.
The only thing you can get public keys out of are X.509 certificates.
Furthermore, there is NO way to export a public key into a PEM-encoded form.

You may need to export a public key from the private key, because the public key provided by the key generated by other tools is in pem format, and we need openssh format
```
<?php
$public = openssl_pkey_get_details(openssl_pkey_get_private(OPENSSL_USER_PRIVATE_KYE))['key'];
// save $public
```

You must also use the string representation of the certificate to get the public key resource:
$dn = array(); // use defaults
$res_privkey = openssl_pkey_new();
$res_csr = openssl_csr_new($dn, $res_privkey);
$res_cert = openssl_csr_sign($res_csr, null, $res_privkey, $ndays);
openssl_x509_export($res_cert, $str_cert);
$res_pubkey = openssl_pkey_get_public($str_cert);