Source

File: wp-includes/functions.php

function wp_normalize_path( $path ) {
	$wrapper = '';

	if ( wp_is_stream( $path ) ) {
		list( $wrapper, $path ) = explode( '://', $path, 2 );

		$wrapper .= '://';
	}

	// Standardise all paths to use '/'.
	$path = str_replace( '\\', '/', $path );

	// Replace multiple slashes down to a singular, allowing for network shares having two slashes.
	$path = preg_replace( '|(?<=.)/+|', '/', $path );

	// Windows paths should uppercase the drive letter.
	if ( ':' === substr( $path, 1, 1 ) ) {
		$path = ucfirst( $path );
	}

	return $wrapper . $path;
}