Form should show alert if duplicate username - php

Question

ive created a form and im using php to check if the username exists in the database, if it is then the form will not submit and echo an alter that the username has already been taken, if i fill the form out and submit it with a name thatn i know is in the database, i get the error saying that the username already exists, but i also get the text that says.. Thank you for signing up
basically its showing the error correctly but it still submitting all the for data to the database
can you see anything in my code thats causing this??
//Check to see if the username is already taken
$query = "SELECT * FROM clients";
$result = mysql_query($query) or die(mysql_error()); // Get an array with the clients
while($row = mysql_fetch_array($result)){ // For each instance, check the username
if($row["username"] == $username){
$usernametaken = true;
}else{$usernametaken = false;}
}
// If the username is invalid, tell the user.
// Or else if the password is invalid, tell the user.
// Or else if the email or PayPal address is invalid, tell the user.
// Else, if everything is ok, insert the data into the database and tell
// the user they’ve successfully signed up.
if($usernametaken)
{
echo "That username has been taken.";
}
if(!preg_match('/^[a-zA-Z0-9]+$/', $username ))// If our username is invalid
{
echo "The username can only contain letters or numbers"; // Tell the user
}
else if(!preg_match('/^[a-zA_Z0-9]+$/', $password ))// If our password is invalid
{
echo "The password can only contain letters or numbers"; // Tell the user
}
// If our email or PayPal addresses are invalid
else if(!preg_match("/^[_a-z0-9-]+(.[_a-z0-9-]+)*#[a-z0-9-]+(.[a-z0-9-]+)*(.[a-z]{2,3})$/", $email))
{
return "The email or PayPal address you entered is invalid."; // Tell the user
}
else{
// Inserts the data into the database
$result = mysql_query("INSERT INTO clients (client_ID, username, password, email, paypal)"."VALUES ('', '$username', '$pw', '$email', '$paypal')");
echo "Thank you for signing up.";
}
?>

Answer 1

You need to break out of your function if the username is no good. You could add an else if before the preg match if you don't want your last line to run. Basically your program flow is
//if username taken
//if bunch of cases
//else add client
There is nothing separating your two if statements.
Your SQL statement is a bear too. You are looping through every client in your db to see if it is a duplicate. Just add a where statement
$query = "SELECT * FROM clients WHERE clientName = '$clientName'";

Answer 2

You don't have anything to tell the script to stop executing if it finds that the username is taken. Restructure your if-else statement like this:
if($usernametaken)
{
echo "That username has been taken.";
} else {
// If username is not taken...
}

Answer 3

Instead of using several else ifs may I recommend using exceptions for this situation.
Demonstration:
<?php
try
{
if ($usernametaken) throw new Exception('That username has been taken.');
// If we've reached here we know data has been checked properly
$result = mysql_query("INSERT INTO clients (client_ID, username, password, email, paypal)"."VALUES ('', '$username', '$pw', '$email', '$paypal')");
echo "Thank you for signing up.";
}
catch (Exception $e)
{
echo $e->getMessage();
}
?>