Vanessa Vasile's Blog: Simple Way to Parse an x509 Certificate with PHP
PHP has a nifty little function for parsing an x.509 SSL certificate into an array to easily pull out the elements: openssl_x509_parse. Essentially, all you need to do is load up the contents of the certificate, either through a file or POST value, and enclose it in the array.
She includes a code snippet that pulls in the certificate contents, calls the function and puts it into an array that's easy to loop through and get values and subvalues from the results. The PHP.net manual page doesn't have a documented list of the return values because a constant result hasn't been defined. You'll need at least PHP 5.2 to use the function.