1st attempt at plugin/shortcodes" FAIL

I tried my hand at making a small plugin that basically has one shortcode command in it. I do a lot of copying and pasting on my site and wanted to simplify the process; but, I’m getting header errors and when I activate the plugin, it messes up my events_planner plugin, saying that headers have already been sent. I have looked for some more info; but, nothing has helped thusfar.

Here’s my code:

<?php
/* Plugin Name: Unsigned
Version: 0.1
Description: Testing out plugin development
Author: Christian King
Author URI: https://www.theunsignedsounds.com
Plugin URI: https://theunsignedsounds.com */?>

<?php
//[unreview] TEMPLATE SHORTCODE
function unreview_func( $atts ){

	extract( shortcode_atts( array(
		'artist'=> 'artistname',

		), $atts ) );

	$audiolink = "https://theunsignedsounds.com/audio/".$artist.".mp3";
	$audio = "[audio src='".$audiolink."']";

	echo $audio;
}
add_shortcode( 'unreview', 'unreview_func' );
?>