WordPress Localized Scripts: Pass PHP data to JavaScript - The PHP code.

functions.php

<?php

  // Place this code in your child theme's functions.php file

  function myprefix_enqueue_scripts() {
        wp_enqueue_script( 
            'custom-script',
            get_stylesheet_directory_uri() . '/js/custom.js'
        );
        wp_localize_script( 'custom-script', 'php_data', array(
                'message' => __('A message that can be translated!' )
            )
        );
  }
  
  add_action('wp_enqueue_scripts', 'myprefix_enqueue_scripts');