openssl_get_privatekey() parses
key and prepares it for use by other functions.
key
key can be one of the following:
a string having the format file://path/to/file.pem. The named file must contain a PEM encoded certificate/private key (it may contain both).
A PEM formatted private key.
passphrase
The optional parameter passphrase must be used
if the specified key is encrypted (protected by a passphrase).