Source

File: wp-admin/includes/class-wp-filesystem-direct.php

public function owner( $file ) {
		$owneruid = @fileowner( $file );

		if ( ! $owneruid ) {
			return false;
		}

		if ( ! function_exists( 'posix_getpwuid' ) ) {
			return $owneruid;
		}

		$ownerarray = posix_getpwuid( $owneruid );

		if ( ! $ownerarray ) {
			return false;
		}

		return $ownerarray['name'];
	}