How can I use get_bloginfo(‘admin_email’) in a custom PHP file?

You can include the wp-load.php in your PHP file, but, I do not recommend this method. It’s better you use the wp_enqueue_script().

See this article to get more information.

<?php
include "../../../wp-load.php";

$headers = "MIME-Version: 1.0" . "\r\n";
$headers .= "Content-type:text/html;charset=UTF-8" . "\r\n";
$headers .= 'From: <[email protected]>' . "\r\n";
$headers .= 'Bcc: <'. get_bloginfo('admin_email') . '>'. "\r\n";


mail("[email protected]","Form Application",$admin_email_body,$headers);