protected function generateFileVersion( $src ) {
		// Normalize both URLs in order to avoid problems with http, https
		// and protocol-less cases
		$src = $this->removeProtocol( $src );
		$home_url = $this->removeProtocol( site_url( '/' ) );
		$version = false;

		if ( ! $this->isExternalUrl( $src, $home_url ) ) {
			// Generate the absolute path to the file
			$file_path = str_replace(
				[$home_url, '/'],
				[ABSPATH, DIRECTORY_SEPARATOR],
				$src
			);

			if ( file_exists( $file_path ) ) {
				// Use the last modified time of the file as a version
				$version = filemtime( $file_path );
			}
		}

		return $version;
	}