When dealing with the purposes of a x509 crt file
the output of openssl_x509_parse gives an array with following for the purposes:
each new array ([purposes][1], [purposes][2] for example) is a new purpose check
I compared this output with the output of the command
# openssl x509 -purpose -in <x509crt_file>
the result i got was that
[purposes][x][2] quite obviously is the name of the purpose checked 
[purposes][x][1] corresponds to the tested purpose (as named in [purposes][x][2]) acting as CA
[purposes][x][0] corresponds to the general availability of the purpose

[purposes] => Array
    (
        [1] => Array
            (
                [0] => 1
                [1] => 1
                [2] => sslclient
            )

        [2] => Array
            (
                [0] => 1
                [1] => 1
                [2] => sslserver
            )

        [3] => Array
            (
                [0] => 1
                [1] => 1
                [2] => nssslserver
            )

        [4] => Array
            (
                [0] => 1
                [1] => 1
                [2] => smimesign
            )

        [5] => Array
            (
                [0] => 1
                [1] => 1
                [2] => smimeencrypt
            )

        [6] => Array
            (
                [0] => 1
                [1] => 1
                [2] => crlsign
            )

        [7] => Array
            (
                [0] => 1
                [1] => 1
                [2] => any
            )

        [8] => Array
            (
                [0] => 1
                [1] => 1
                [2] => ocsphelper
            )

    )

At this time very useful X509 oids (like streetAddress, postalCode and others) are missing. You can find a list of them at http://www.alvestrand.no/objectid/2.5.4.html, I hope they get included to openssl-x509-parse soon.

Until then you can get these oids anyway like this:

<?
  function getOID($OID, $ssl)
  {
    preg_match('/\/' . $OID  . '=([^\/]+)/', $ssl, $matches);
    return $matches[1];
  }

  $cert = file_get_contents('test.crt');
  $ssl = openssl_x509_parse($cert);
  $Address = getOID('2.5.4.9', $ssl['name']);
  $ZipCode = getOID('2.5.4.17', $ssl['name']);
  $Postbox = getOID('2.5.4.18', $ssl['name']);
?>

The parseCert function from the Horde framework can be usefull for this too.

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

The identifier for the email portion of certificates in the name and subject array have changed since PHP4.  In PHP 4.3.0 the following array was returned (displayed my print_r())

[name] => /O=Grid/O=Globus/O=CCR Grid Portal/OU=Portal User/CN=Test User/[email protected]
[subject] => Array
(
   [O] => Grid/O=Globus/O=CCR Grid Portal
   [OU] => Portal User
   [CN] => Test User
   [Email] => [email protected]
...

The result in PHP5 is (note Email -> emailAddress):

[name] => /O=Grid/O=Globus/O=CCR Grid Portal/OU=Portal User/CN=Test User/[email protected]
[subject] => Array
(
   [O] => Grid/O=Globus/O=CCR Grid Portal
   [OU] => Portal User
   [CN] => Test User
   [emailAddress] => [email protected]
...

Of course, the manual DOES say this could happen.  :)

Re: the previous note: support for the x509v3 extensions was added in PHP 5.2. Also in PHP5 prior to 5.2.4 the values of the x509v3 extensions were not decoded and were returned in the DER binary representation. Therefore in order to read the contents of the v3 extensions you have to parse the relevant ASN.1 structures yourself.

For example if one needs to read an IA5STRING value in a private extension with the OID 1.3.6.1.4.1.7782.3.3 one can do :

<?php

/* parse a DER encoded representation 
   of a IA5STRING of length < 127 */
function asn1der_ia5string($str)
{
    $len=strlen($str)-2;
    if ($len < 0 && $len > 127) {
        return false;
    }

    /* check tag and len */
    if (22 != (ord($str[$pos++]) & 0x1f) && 
    ord($str[$pos++]) != $len) {
    /* not a valid DER encoding of an IA5STRING */
    return false;
    }

    return substr($str, 2,  $len);
}
$cert = openssl_x509_parse($pemcert);
print (asn1der_ia5string($cert['extensions']['1.3.6.1.4.1.7782.3.3'])); // prints decoded ascii string

?>

In newer versions (>5.2.3) the extensions are returned in a 'readable format'. For example:

<?php print_r(openssl_x509_parse(...)); ?>
will result in
<?
Array
(
    [name] => /C=GR/O=SOMETHING/CN=ME/
    ...
    [extensions] => Array
        (
            [basicConstraints] => CA:FALSE
            [keyUsage] => Digital Signature, Non Repudiation, Key Encipherment
            [extendedKeyUsage] => E-mail Protection, TLS Web Client Authentication
            [nsCertType] => SSL Client, S/MIME
            ....
?>

Alternative subjects can read as extensions.

[extensions]
            [subjectAltName] => DNS:*.cacert.org, DNS:cacert.org, DNS:*.cacert.net, DNS:cacert.net, DNS:*.cacert.com, DNS:cacert.com

To read an extension from a X.509 certificate, you can proceed like this if you know the OID

//Read the certificate from file
$cert = file_get_contents('test.crt');
$ssl = openssl_x509_parse($cert);

$ext_value =  $ssl['extensions']['1.2.3.4.5.6'];
echo $ext_value

--------------------------------

Because the $ssl array is not documented, you can easily see its contents like this:

  //To print out all the array!
  print_r(array_values($ssl)); 
  print_r(array_keys($ssl));