验证客户端证书上列出的电子邮件

I am writing a front end app for my company using PHP on Apache server.

Everyone in my company have their own client certificates.

I can use below code

SSLVerifyClient require
SSLVerifyDepth 10
SSLOptions +StdEnvVars  
SSLCACertificateFile "C:/xampp/htdocs/frontEnd/clientca.crt"

to force the user to select the client certificate they want to use, if their client certificate was signed by one of the intermediate certificate listed in the file.

I am trying to add another layer of security by checking the email listed on the certificate. For example, I only allow the client certificate with an email under @example.com to access the site.

I've checked a lot of websites for example mod_ssl but I still can't get my head around it.

Can anyone please help me?

Thank you.