I have some troubles to find the right loop to check if some values are contained in mysql DB. I'm making a software and I want to add license ID. Each user has x keys to use.

Now when the user start the client, it invokes a PHP page that check if the Key sent in the POST method is stored in DB or not.

If that key isn't store than I need to check the number of his keys. If it's > than X I'll ban him otherwise i add the new keys in the DB.

I'm new with PHP and MYSQL. I wrote this code and I would know if I can improve it.

<?php

$user = POST METHOD 
$licenseID = POST METHOD

$resultLic= mysql_query("SELECT  id , idUser , idLicense FROM license WHERE idUser = '$user'") or die(mysql_error());
$resultNumber = mysql_num_rows($resultLic);
$keyFound = '0'; // If keyfound is 1 the key is stored in DB

while ($rows = mysql_fetch_array($resultLic,MYSQL_BOTH)) {
   //this loop check if the $licenseID is stored in DB or not
    for($i=0; $i< $resultNumber ; i++) { 
        if($rows['idLicense'] === $licenseID) {
            //Just for the debug 
            echo("License Found");
            $keyFound = '1';
            break;
    }
    //If key isn't in DB and there are less than 3 keys the new key will be store in DB

    if($keyfound == '0' && $resultNumber < 3) { 
        mysql_query( Update users set ...Store $licenseID in Table) 
    }

    // Else mean that the user want user another generated key (from the client) in the DB and i will be ban (It's wrote in TOS terms that they cant use the software on more than 3 different station)                    
    else { 
        mysql_query( update users set ban ='1'.....etc );
    }                               
}

?>          

I know that this code seems really bad so i would know how i can improve it. Someone Could give me any advice?

I choose to have 2 tables: users where all information about the users is, with fields id, username, password and another table license with fields id, idUsername, idLicense (the last one store license that the software generate)