How do i set global variable in a function in wordpress functions.php

Question

Please vet this code and help me with how to declare '.($a['link']).' variable such that i can echo its content in single.php

function mp3_download_link_att($atts, $content = null) {
    $default = array(
        'link' => '#',
    );
    session_start();
    global $post;
    $title = get_the_title($post->ID);
    $a = shortcode_atts($default, $atts);
    $content = do_shortcode($content);

    return ' <h2>Download '.$title.' Mp3 Audio</h2><p style="text-align: center;"><strong>Download Mp3 audio, listen and share this amazing song for free and stay happy.</strong></p><figure class="wp-block-audio"><audio class="audioPlayer" src="'.($a['link']).'" controls="controls"></audio></figure><center><p class="song-download"><a rel="nofollow" id="dlf" class="button download" href="'.($a['link']).'"><span class="fa fa-download"></span> DOWNLOAD '.$content.' MP3 <span class="fa fa-music"></span> </a></p></center>';
}
$_SESSION['myduration'] = ($a['link']);
add_shortcode('mp3download', 'mp3_download_link_att');

Answer 1

Declare $a as a global variable, just as you have with $post:

global $post, $a;

(Keep in mind, you may collide with other variables, since your var name is not really all that unique)

Answer 2

Either declare your global variable outside of function scope (aka, global scope), or globalize it with the global keyword.

function foobar() {
  global $aduration;
  $aduration = shortcode_atts($default, $atts);
  //...
}

To access a global you declared elsewhere, use the global keyword again (globalize the variable), just like you do with global $post in your original code:

// Assuming `aduration` is declared global somewhere else, like shown above:
<?php global $aduration; echo $aduration['link']; ?>

In short, preceding any usage of your global variable with global $myglobal indicates that this is indeed a global variable - regardless of whether you're declaring it or accessing it. If you declare it inside a function in functions.php without the global keyword, it's nothing more than just another local variable, inside that function.

If declared properly as a global in functions.php, and want to access it somewhere else - such as in single.php - globalizing it effectively imports it, so php knows to check the global scope.