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 — 将 PKCS#12 兼容证书存储文件导出到变量
$certificate,&$output,$private_key,$passphrase,$options = []
   openssl_pkcs12_export() 以 PKCS#12 文件格式将 certificate 导入到名为 output 的字符串变量中。
  
| 版本 | 说明 | 
|---|---|
| 8.0.0 | 
       certificate 现在接受 OpenSSLCertificate
       实例;之前接受类型 OpenSSL X.509 CSR 的 resource。
       | 
     
| 8.0.0 | 
       private_key 现在接受 OpenSSLAsymmetricKey
       或 OpenSSLCertificate 实例;之前接受类型 OpenSSL key
       或 OpenSSL X.509 的 resource。
       | 
     
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);?>