How to export the private key from a Salesforce Self Signed cert using PHP or openssl?

Question

I created a Self-Signed cert by following this article I set the private key as exportable but there isn't an export link within the salesforce app (that I can see) so I'm guessing you have to export from the certificate itself. I'm using the PHP openssl x509 functions but i can't get it to work. I keep getting openssl_sign(): supplied key param cannot be coerced into a private key... when I run this code:

...
$private_key = openssl_get_privatekey(file_get_contents(env('SALESFORCE_CERT_FILE')));
$s = "";
openssl_sign($header . '.' . $payload, $s, $private_key, "SHA256");
...

Answer 1

I figured it out. In salesforce they do have an "export" button that says "Export to Keystore". I was unfamiliar with this so I didn't think to use it. I was looking for export private key or something like that. Turns out you can just following the answer to this stack exchange question to get your private key.