It is not clear whether or not you are using mod_proxy or if your PHP application is making the SSL connection directly.
If you are using mod_proxy then you need to use the following directives in your Apache http config, most likely in the appropriate Location block:
Normally ca.pem contains the certificates of the CAs you trust to sign server certificate but since you are using a self-signed certificate ca.pem will contain the self-signed certificate.
The client.pem file contains the client key and certificate in PEM format (BEGIN CERTIFICATE ... END CERTIFICATE) concatenated together, with the key first.
If you are making the SSL connection directly from your PHP code then Apache httpd is not directly involved and just runs your PHP script.