PHP - How can i keep my user logged in?

I'm using a function for login.

But when i change the page from my website, i have to login again.

how can i keep my user logged in when i change my page?

Here my code:

<?php
error_reporting(0);
if($_POST['login']=="") {
?>
  <form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>" >
    <label><a>Utilizador</a><input type="text" name="login" id="s-user"></label>
    <label><a>Senha</a><input type="text" name="password" id="s-pass"></label>
      <input type="submit" class="submit" value="Entrar">

  </form>


<?php
}
else {
?>
<?php
include("classes/Utilizadores/Cliente.class.php");


        if($_REQUEST['login']!="") {
            if($_REQUEST['password']!="") {

                            $clientes = new Cliente();
                            if($clientes->verificarCliente($_REQUEST['login'], $_REQUEST['password'])) {
                                echo "<br>";
                            } else {
                                echo "<br><a>Login ou senha errados, caso não tenha, <a href='criarconta.php'> registre-se</a>, <a>ou</a> <a href='index.php'>volte a tentar.</a></a><br>";
                            }
                            $clientes->endCliente();

            } else {
                echo "ERRO: Deve introduzir a sua password...<br>";
            }
        } else {
            echo "ERRO: Deve introduzir o seu login...<br>";
        }

}
?>

My function code:

function verificarCliente($login, $password) {
    $sql = "SELECT * FROM users WHERE login LIKE '$login' AND password LIKE '$password'";
    if(($rs=$this->bd->executarSQL($sql))){
        if(mysql_fetch_row($rs)==false) {
            return false;
        } else {

                                echo "<br><b> <a>Bem-Vindo <font size=2>" .mysql_result($rs,0,"login")."</font></b></a><br><br><br>";     

            return true;

        }
    }
    else {
        return false;
    }
}