gistfile1.php

<?php

add_action('wp', function() {

	if( '1' == get_query_var('se_random_post') ) {

		$count = count( get_posts() );

		if( (int) $count <= 0 )
			return; // no posts

		$picked = rand( 0, abs( $count - 1 ) );
		$picked = (int) get_posts()[$picked]->ID;

		if( get_post( $picked ) ) {
			wp_redirect( get_the_permalink( $picked ) );
			exit;
		} else {
			// try again (this may cause a redirect loop if no success pulling a random post)
			wp_redirect( home_url('/random/') );
			exit;
		}

	}

});