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<?php

?>
function registerUser() {
   mysql_connect('server', 'username', 'password', 'database');
   $username = $_POST['username'];
   $password = md5($_POST['password']);

   $sql = "INSERT INTO tblUsers (fldUsername, fldPassword) VALUES ($username, $password);";

   mysql_query($sql);
}
<html>
   <head>
       <title>Registration</title>
   </head>
   <body>
       <form action="<?php $_SERVER['PHP_SELF']."?register=true" ?>" method="post">
           Username: <input type="text" name="username">
           Password: <input type="password" name="password">
           <input type="submit" value="Register">
       </form>
   </body>
</html>
<?php
function registerUser() {
   mysql_connect('server', 'username', 'password', 'database');
   $username = $_POST['username'];
   $password = md5($_POST['password']);

   $sql = "INSERT INTO tblUsers (fldUsername, fldPassword) VALUES ($username, $password);";

   mysql_query($sql);
}
?>

<html>
   <head>
       <title>Registration</title>
   </head>
   <body>
       <form action="<?php $_SERVER['PHP_SELF']."?register=true" ?>" method="post">
           Username: <input type="text" name="username">
           Password: <input type="password" name="password">
           <input type="submit" value="Register">
       </form>
   </body>
</html>
<?php
if ($_GET['register'] == 'true') {
   registerUser();
}

function registerUser() { ....