# Convert the .p12 file into a .pem file:
$ openssl pkcs12 -nocerts -in Filename.p12 -out FileName.pem

# Convert the .cer file into a .pem file:
$ openssl x509 -in apns_development.cer -inform der -out PushCert.pem

# Finally, combine the certificate and key into a single .pem file
$ cat PushCert.pem FileName.pem > final.pem

# Finally use final.pem or FileName.pem