Place this file in wp-content/mu-plugins/ to patch missing wp_parse_url() function

wp-parse-url-mu-plugin.php

<?php 
if( ! function_exists( 'wp_parse_url' ) ) {
	function wp_parse_url( $url, $component = -1 ) {
		$to_unset = array();
		$url = strval( $url );

		if ( '//' === substr( $url, 0, 2 ) ) {
			$to_unset[] = 'scheme';
			$url = 'placeholder:' . $url;
		} elseif ( '/' === substr( $url, 0, 1 ) ) {
			$to_unset[] = 'scheme';
			$to_unset[] = 'host';
			$url = 'placeholder://placeholder' . $url;
		}

		$parts = @parse_url( $url );

		if ( false === $parts ) {
			// Parsing failure.
			return $parts;
		}

		// Remove the placeholder values.
		foreach ( $to_unset as $key ) {
			unset( $parts[ $key ] );
		}

		return _get_component_from_parsed_url_array( $parts, $component );
	}

	if( ! function_exists( '_get_component_from_parsed_url_array' ) ) {
		function _get_component_from_parsed_url_array( $url_parts, $component = -1 ) {
			if ( -1 === $component ) {
				return $url_parts;
			}

			$key = _wp_translate_php_url_constant_to_key( $component );
			if ( false !== $key && is_array( $url_parts ) && isset( $url_parts[ $key ] ) ) {
				return $url_parts[ $key ];
			} else {
				return null;
			}
		}
	}

	if( ! function_exists( ' _wp_translate_php_url_constant_to_key' ) ) {
		function _wp_translate_php_url_constant_to_key( $constant ) {
			$translation = array(
				PHP_URL_SCHEME   => 'scheme',
				PHP_URL_HOST     => 'host',
				PHP_URL_PORT     => 'port',
				PHP_URL_USER     => 'user',
				PHP_URL_PASS     => 'pass',
				PHP_URL_PATH     => 'path',
				PHP_URL_QUERY    => 'query',
				PHP_URL_FRAGMENT => 'fragment',
			);

			if ( isset( $translation[ $constant ] ) ) {
				return $translation[ $constant ];
			} else {
				return false;
			}
		}
	}
}