If you want to include CA-Certificates in the PKCS12 it can be accomplished by using the $args parameter.
<?php
$args = array(
'extracerts' => $CAcert,
'friendly_name' => 'My signed cert by CA certificate'
);
openssl_pkcs12_export($signed_csr, $cerificate_out, $private_key_resource, $passphrase, $args);
?>