title: PHP Login application - Check if username already exists when creating a new user
body: <p>The title tells pretty much everthing, that needs to be said. I'm having a registration code looking like this: </p>
  
  <p>I wan't it to check, if the username entered already exists, if it does - write and $errMsg = ""; and echo it out later.. I hope you can help me, thanks. </p>
  
  <pre><code>if(isset($_POST['username']) &amp;&amp; isset($_POST['password']) &amp;&amp; isset($_POST['name']) &amp;&amp; isset($_POST['last_name']) &amp;&amp; isset($_POST['company'])){
  
              if($username === '') {
                  $errMsg = "Du skal udfylde brugernavn";
              }
              elseif($password === ''){
                  $errMsg = "Du skal udfylde password";
              }
              elseif($name === ''){
                  $errMsg = "Du skal udfylde navn";
              }
              elseif($last_name === ''){
                  $errMsg = "Du skal udfylde efternavn";
              }
              elseif($company === ''){
                  $errMsg = "Du skal udfylde firma";
              }
                  $sql = ("SELECT * FROM members WHERE username ='$username'");
                  $result = mysql_query($sql) or die('error');
                  $row = mysql_fetch_assoc($result);
                      if(mysql_num_rows($result)) {
                              $errMsg = 'Brugernavn findes, vælg et andet.';
                  } else {
                  $sql = ("INSERT INTO members (username, password, name, last_name, company, salt)VALUES('$username', '$password', '$name', '$last_name', '$company', '$salt')")or die(mysql_error());
  
                  if(mysql_query($sql))
                  echo "Du er oprettet som profil.";
                  }
  
              }//End whole if 
  </code></pre>