Converting .cer to .pem via pure PHP, (no system, backticks, shell_exec, exec, etc.) to get the same result as with "openssl x509 -inform der -in cert.cer -out cert.pem". Note, I am not expert on certificates, etc. This specific certificate conversion simply worked for me.

cerToPem.php

<?php

$certificateCAcer = '/certificate.cer';

$certificateCAcerContent = file_get_contents($certificateCAcer);

/* Convert .cer to .pem, cURL uses .pem */
$certificateCApemContent =  '-----BEGIN CERTIFICATE-----'.PHP_EOL
    .chunk_split(base64_encode($certificateCAcerContent), 64, PHP_EOL)
    .'-----END CERTIFICATE-----'.PHP_EOL;

$certificateCApem = $certificateCAcer.'.pem';

file_put_contents($certificateCApem, $certificateCApemContent); 

Thanks for your help!
I need to do the same but to convert file.key to "file.key.pem".. May you help me?

Thxxx Helped me with Apple Certification

You saved my life,
Thank you, sir...

Muchas gracias por el aporte. Me ha servido mucho :)

Thank you for your sharing

thank you for sharing,it's very useful to me

Thanks for your code, is really useful.

Thank you, mate!