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);