To get the real timestamps as integer values for the validity daterange you can use as follows:
<?php
$data = openssl_x509_parse(file_get_contents('/path/to/cert.crt'));
$validFrom = date('Y-m-d H:i:s', $data['validFrom_time_t']);
$validTo ) date('Y-m-d H:i:s', $data['validTo_time_t']);
echo $validFrom . "\n";
echo $validTo . "\n";
?>