The valid from/to info is returned twice, in two different formats. They can be converted to normal datetime objects like this:
$x509_data = openssl_x509_parse($cert);
date_create_from_format('ymdHise', $x509_data['validFrom'])->format('c');
date_create( '@' . $x509_data['validFrom_time_t'])->format('c');
/* these give the same result */
To get a human-readable format directly (or any other formatted string) instead of a datetime object, use this:
date_create_from_format('ymdHise', $x509_data['validFrom'])->format('c');
or
date_create( '@' . $x509_data['validFrom_time_t'])->format('c');
The same applies to validTo and validTo_time_t
