title: How to convert P7C file to pem file using PHP7.1?
body: <p>I am dealing with some certificate problems.
  I want to convert a p7c file to pem file.
  For the safety reason, the system has disable some php functions such as "system, exec" and so on.
  But the openssl extension in php is not complete implementation, so I don't how to do it.
  Can somebody help me? Thanks.</p>