As you probably found, getting the public key is not as direct as you might think with this documentation.
You can easily get into messages like:
Warning: openssl_pkey_get_public(): Don't know how to get public key from this private key (the documentation lied) in D:\www\keys.php on line 4
The correct steps to get the whole thing seem to be these:
$dn = array("countryName" => 'XX', "stateOrProvinceName" => 'State', "localityName" => 'SomewhereCity', "organizationName" => 'MySelf', "organizationalUnitName" => 'Whatever', "commonName" => 'mySelf', "emailAddress" => 'firstname.lastname@example.org');
$privkeypass = '1234';
$numberofdays = 365;
$privkey = openssl_pkey_new();
$csr = openssl_csr_new($dn, $privkey);
$sscert = openssl_csr_sign($csr, null, $privkey, $numberofdays);
openssl_pkey_export($privkey, $privatekey, $privkeypass);
echo $privatekey; echo $publickey; echo $csrStr; ?>
Now all you need to do is to make some research on each individual function.