While trying to update a certificate on a produciton enviroment I ran into the following issue, Apache only accepts certificates in PEM format, since my client provided me with his new certificate on a .cer file I had to find a way to convert it.
For x509 certificates this can be easily achieved using OpenSSL on Linux/Unix and OS X. First make sure you have OpenSSL (comes by default with OS X), open a terminal and issue the following command:
openssl x509 -inform pem -in certificate.cer -outform der -out certificate.pem
Where certificate.cer is the source certificate file you want to convert and certificate.pem is the name of the converted certificate.