check & insert

[gesf]

You can do something like (example):

<?php

$usr = $_POST['username']; 

$exists = mysql_query("SELECT username FROM users WHERE username = '$usr'");

if(mysql_num_rows($exists) != 0) {
   // A match was found
   // return error
}
else {
   // username does not exists, so let's insert it
   mysql_query("INSERT INTO users SET username = '$usr'");
}

?>