Working php session logic (changes made in functions.php file)

functions.php

function vg_show_post_id() {
	$p_id = $_REQUEST['post_id'];
	$title = get_the_title( $p_id );
	if ( defined( 'DOING_AJAX' ) && DOING_AJAX ) {

			if (!empty($_SESSION['cart']) && array_key_exists( $p_id, $_SESSION['cart'] ) ) {

				$cnt = $_SESSION['cart'][$p_id];
				$cnt++;
				$_SESSION['cart'][$p_id] = $cnt;

			} else {

				$_SESSION['cart'][$p_id] = 1;

			}

			echo '<pre>';
			print_r( $_SESSION['cart'] );

			echo '</pre>';
			die();

	} else {
		echo "Not in admin-ajax";
	}

}
add_action( 'wp_ajax_nopriv_vg_show_post_id', 'vg_show_post_id' ); // for ajax
add_action( 'wp_ajax_vg_show_post_id', 'vg_show_post_id' ); // for ajax