Simplest of plugins doesnt work even though every step is followed

Trying to learn WP plugin development.

Followed all the instructions.

Main php file:

<?php
/**
 * Plugin Name: Test 1
 * Plugin URI: http://xxxxx.xxx
 * Description: xxxxxxxxxxxxxxxxxx
 * Version: 1.0.0
 * Author: xxxxxxx
 * Author URI: http://xxxxxx.xxx
 * License: GPL2
 */

My one an only php file with code:

<?php

function testhook() {
    $output="<script> alert('Page is loading...'); </script>";
    echo $output;
echo 'SOULISSSSSSSSSSSS';
}
add_action( "wp_head", "testhook" );

It simply does nothing. View Source doesnot contain the string anywhere.

Of course i have activated the plugin, cleared caches e.t.c

Obviously my noobness stops me from finding the mistake