<?php
session_start();

$data=array("user1"=>array("url"=>"page1.php","password"=>"12345"),
            "user2"=>array("url"=>"page2.php","password"=>"12345"),
        "user3"=>array("url"=>"page3.php","password"=>"12345")
        );

if(isset($_POST['username']) && isset($_POST['password'])) {

    $user = strtolower($_POST['username']);
    $pass = strtolower($_POST['password']);

    if($data[$user]['password'] == $pass) {
       $_SESSION['username'] = $user . " " . $pass;
       header('Location: ' . $data[$user]['url']);
    } else {
        echo '<script type="text/javascript">window.onload = function() { document.getElementById("loginError").innerHTML = "' . "Invalid login details. Please try again." . '"; }</script>';
        logIn();
    }
} else {
    logIn();
}
?>

if(isset($_POST['username']) && !empty($_POST['username']) &&
    isset($_POST['password']) && !empty($_POST['password']) ) {
    //Do code here
}
else
{
   logIn();
}

...
$user = strtolower(trim($_POST['username']));
$pass = strtolower(trim($_POST['password']));

if($user !== '' && $pass !== '') {
  //continue with login logic
  if($data[$user]['password'] === $pass) {
  ...
}
else{
//print error
}

session_start();

$data=array("user1"=>array("url"=>"page1.php","password"=>"12345"),
            "user2"=>array("url"=>"page2.php","password"=>"12345"),
            "user3"=>array("url"=>"page3.php","password"=>"12345")
            );

if (!empty($_POST['username'] && !empty($_POST['password'])) {

    $user = strtolower($_POST['username']);
    $pass = strtolower($_POST['password']);

    if (isset($data[$user]) && $data[user]['password'] == $pass) {
        $_SESSION['username'] = $user . ' ' . $pass;
        header("Location: " . $daa[$user]['url']);
    } else {
        echo '<script type="text/javascript">window.onload = function() { document.getElementById("loginError").innerHTML = "' . "Invalid login details. Please try again." . '"; }</script>';
        logIn();
    }

} else {
    logIn();
}