[Ethereum] Generate Ethereum address in PHP

As I said, you coud use JSON rpc to create a wallet and and set password. But PHP and geth don't go well together, it will limit your app. I recommend going directly with web3.js, javascrio/jquery/nodejs.

For generating wallet thought JSON-RPC (pass sent over the network):

//create eth wallet supplying pass, return wallet address if created

function getethwallet($pass) {

    $url = "http://node.ip:8545";   
    $data = array(
                 "jsonrpc" => "2.0",
                 "method" => "personal_newAccount",
                 "params" => array($pass),
                 "id" => "1"
                 );

    $json_encoded_data = json_encode($data);


$ch = curl_init($url);                                                                      
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");                                                                     
curl_setopt($ch, CURLOPT_POSTFIELDS, $json_encoded_data);                                                                  
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);                                                                      
curl_setopt($ch, CURLOPT_HTTPHEADER, array(                                                                          
    'Content-Type: application/json',                                                                                
    'Content-Length: ' . strlen($json_encoded_data))                                                                       
);                                                                                                                   

$result = json_decode(curl_exec($ch));
curl_close($ch);

        $parsed = $result->result;

return $parsed;
}   

Accounts can be generated entirely off-line without ever connecting to the Internet (there are several questions about this on stack exchange), so yes, you could do it without running a node.

For this application, perhaps the best way is to use an hierarchical deterministic (HD) wallet. You can generate and store a single seed used to generate a whole family of addresses (this makes backing things up easier). This avoids having to suspend your service if you don't have time to back up new wallet keys (assuming you have other replication services in place to protect public key-account pairs).

On the other hand, why do you want to keep the private keys for your users? Is this for a service that interacts with the blockchain? In any case, if you're trying to protect users from bad key-storage practices, it might be easier from a management/security perspective to have fewer addresses (though there are arguments for more accounts, too); users can all hook into the same hot/cold wallet. In the case of an exchange, one could argue for a one-to-many or many-to-many relationship for wallets.

For any other purpose where you're not trying to protect users from bad practices, my short brainstorming session says it either doesn't make sense to be using the blockchain (just create a regular centralized service) or have users generate their own accounts and submit the account address to you to store. One of the advantages of Ethereum is "you are your own bank". Relying on someone else also reduces the utility of Ethereum (since users must now manage one more centralized account).