Código PHP:
<?php
$servidor = "localhost";
$usuario = "yosoyun";
$password = "chismoso";
$conexion = mysql_connect($servidor, $usuario, $password) or die("no se pudo conectar a base de datos".mysql_error());
$selec = mysql_select_db("LABD",$conexion);
$usuarios = mysql_query("Select * from users where username = '".$_REQUEST['nombre']."'",$conexion);
$num = mysql_num_rows($usuarios);
$accept = "<img src='objetos/acept.png' alt='revisar' width='16' height='16' border='0' />";
$cancel = "<img src='objetos/cancel.png' alt='revisar' width='16' height='16' border='0' />";
echo "<h5>";
if ($num==0){
echo @$_REQUEST['nombre'].' ok'.' '.$accept.' '.$elresto;
}else{
echo @$_REQUEST['nombre'].' has already been taken '.$cancel;
echo "</h5>";
}
mysql_close($conexion);
?>