Take the input of a "Twitter" field, decide whether it's a handle or a URL, and generate a URL

gistfile1.php

/**
 * Note that it uses trailingslashit(), a WP function. Write your own logic if not using WP
 */
function sanitize_twitter_handle( $handle ) {
		$parts = parse_url( $handle );
		$url = '';

		if ( empty( $parts['host'] ) ) {
			// not a url
			if ( 0 === strpos( $handle, '@' ) ) {
				$handle = substr( $handle, 1 );
			}

			$url = 'https://twitter.com/' . $handle . '/';
		} else {
			$url = trailingslashit( 'https://twitter.com' . $parts['path'] );
		}

		return $url;
	}