Getting Plugin directory and url

Use the plugin_dir_path() and plugin_dir_url() helper functions:

if( ! defined( 'MY_PLUGIN_PATH' ) )
    define( 'MY_PLUGIN_PATH', plugin_dir_path( __file__ ) . 'admin/' );
if( ! defined( 'MY_PLUGIN_DIR' ) )
    define( 'MY_PLUGIN_DIR', plugin_dir_url( __file__ ) . 'admin/' );

You should be using more unique constant names – ADMIN_PATH is far too generic.

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