s-oneill

Create an appropriately named folder and php file
   /wp-content/plugins/pluginname/pluginname.php

Begin the file with an opening <?php tag followed by the standard comment for WP plugins

  / *
  Plugin Name: Plugin Name Script
  Plugin URI: https://s-oneill.com/
  Description: Load scripts for Plugin Name page
  Version: 1.0
  Author: Sean ONeill
  Author URI: https://s-oneill.com
  License: GPLv2 or later
  * /

The Plugin Name will appear in the Admin Plugins area for you to activate when ready.

Create php function(s) to carry out the purpose of the plugin.
Note: php functions must be called (or “hook”ed) into some action in the WP process using a function:

  add_action( 'init', 'add_Cookie' )

  function add_Cookie() {
    setcookie("last_visit_time", date("r"), time()+60*60*24*30, "/");
  }

The add action function has two required parameters,
1) Action name (there are many different WP actions )
2) Name of your function.

  add_action( 
    string $tag, // req. string - the action name
    callable $function_to_add, // req. string - the function to fire 
    int $priority = 10,  // opt. int. - priority/call order, 1 called 1st def 10
    int $accepted_args = 1 // opt. int. - # of args fn accepts, default is 1
  )

(The priority and accepted_args parameters are optional )

Any JS needed in the plugin must be included using:
   wp_enqueue_script()

  define( 'MYSCRIPT_VERSION', 1);
  function our_function_name($hook) {

    wp_enqueue_script( 
      'my-plugin-script-name', // name of script
      plugins_url('js/my-plugin-script-name.js', __FILE__ ), // location
      array( 'jquery' ), // dependencies
      MYSCRIPT_VERSION, // version (declared in a var above)
      true // include script in footer (false will include in header)
    );
  }
  add_action( 'admin_enqueue_scripts', 'our_function_name' )

Note: the JS file can be in a /js folder in your plugin, or external. Be sure to use the proper location in the 2nd param.
Also, __FILE__ tells the plugins URL function that we’re referencing a file in this plugin

Precede the wp_enqueue_script() function with an if() to restrict the script to specific pages

  function our_function_name_w_enqueue_script($hook) {

      if ( !is_page( 'about') )
      return;

     wp_enqueue_script( 
      'my-plugin-script-name', // name of script
      plugins_url('js/my-plugin-script-name.js', __FILE__ ), // location
      array( 'jquery' ), // dependencies
      MYSCRIPT_VERSION, // version (declared in a var above)
      true // include script in footer (false will include in header)
    );
  }
  add_action( 'admin_enqueue_scripts', 'our_function_name' )
}
 add_action( 'admin_enqueue_scripts', 'our_function_name_w_enqueue_script' )