If your certificate is not password-protected, just use null or a blank string.  Otherwise, this function won't work.(PHP 5 >= 5.2.2, PHP 7, PHP 8)
openssl_pkcs12_export — Exports a PKCS#12 Compatible Certificate Store File to variable
$certificate,&$output,$private_key,$passphrase,$options = []
   openssl_pkcs12_export() stores
   certificate into a string named by
   output in a PKCS#12 file format.
  
x509See Key/Certificate parameters for a list of valid values.
outputOn success, this will hold the PKCS#12.
private_keyPrivate key component of PKCS#12 file. See Public/Private Key parameters for a list of valid values.
passphraseEncryption password for unlocking the PKCS#12 file.
optionsOptional array, other keys will be ignored.
| Key | Description | 
|---|---|
| "extracerts" | array of extra certificates or a single certificate to be included in the PKCS#12 file. | 
| "friendly_name" | string to be used for the supplied certificate and key | 
| Version | Description | 
|---|---|
| 8.0.0 | certificateaccepts an OpenSSLCertificate instance now;
       previously, a resource of typeOpenSSL X.509 CSRwas accepted. | 
| 8.0.0 | private_keyaccepts an OpenSSLAsymmetricKey
       or OpenSSLCertificate instance now;
       previously, a resource of typeOpenSSL keyorOpenSSL X.509was accepted. | 
If your certificate is not password-protected, just use null or a blank string.  Otherwise, this function won't work.If you need to provide multiple additional certificates, the 'extracerts' argument needs to be an array with one certificate per element:
<?php
$args = array(
    'extracerts' => array(
        0 => '-----BEGIN CERTIFICATE----- cert1 ...',
        1 => '-----BEGIN CERTIFICATE----- cert2 ...',
        // ...
        )
    );
?>
You can use this to prepare a PEM.
<?php
$pemChain = '...';
preg_match_all('/(-----BEGIN CERTIFICATE-----.*?-----END CERTIFICATE-----)/si', $pemChain, $matches);
$args = array('extracerts' => $matches[0]);
openssl_pkcs12_export($signed_csr, $cerificate_out, $private_key_resource, $passphrase, $args);
?>in order to export a private key to pkcs12 format, the input certificate must contain both private and associated public key in PEM format ,  
-----BEGIN RSA PRIVATE KEY-----
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----
else this function might return the following error "openssl_pkcs12_export(): cannot get cert from parameter 1"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);
?>