获取一个sanitized文件名，该文件名对给定目录来说是唯一的。

如果文件名不是唯一的，那么一个数字将被添加到文件名的在扩展名之前，并将继续添加数字，直到文件名是唯一的。

回调函数允许调用者使用他们自己的方法来创建唯一的文件名。如果定义了回调，回调应该接受三个参数。
– 目录、基本文件名和扩展名 – 并返回一个唯一的文件名。

function wp_unique_filename( $dir, $filename, $unique_filename_callback = null ) {
	// Sanitize the file name before we begin processing.
	$filename = sanitize_file_name( $filename );
	$ext2     = null;

	// Initialize vars used in the wp_unique_filename filter.
	$number        = '';
	$alt_filenames = array();

	// Separate the filename into a name and extension.
	$ext  = pathinfo( $filename, PATHINFO_EXTENSION );
	$name = pathinfo( $filename, PATHINFO_BASENAME );

	if ( $ext ) {
		$ext = '.' . $ext;
	}

	// Edge case: if file is named '.ext', treat as an empty name.
	if ( $name === $ext ) {
		$name = '';
	}

	/*
	 * Increment the file number until we have a unique file to save in $dir.
	 * Use callback if supplied.
	 */
	if ( $unique_filename_callback && is_callable( $unique_filename_callback ) ) {
		$filename = call_user_func( $unique_filename_callback, $dir, $name, $ext );
	} else {
		$fname = pathinfo( $filename, PATHINFO_FILENAME );

		// Always append a number to file names that can potentially match image sub-size file names.
		if ( $fname && preg_match( '/-(?:d+xd+|scaled|rotated)$/', $fname ) ) {
			$number = 1;

			// At this point the file name may not be unique. This is tested below and the $number is incremented.
			$filename = str_replace( "{$fname}{$ext}", "{$fname}-{$number}{$ext}", $filename );
		}

		/*
		 * Get the mime type. Uploaded files were already checked with wp_check_filetype_and_ext()
		 * in _wp_handle_upload(). Using wp_check_filetype() would be sufficient here.
		 */
		$file_type = wp_check_filetype( $filename );
		$mime_type = $file_type['type'];

		$is_image    = ( ! empty( $mime_type ) && 0 === strpos( $mime_type, 'image/' ) );
		$upload_dir  = wp_get_upload_dir();
		$lc_filename = null;

		$lc_ext = strtolower( $ext );
		$_dir   = trailingslashit( $dir );

		/*
		 * If the extension is uppercase add an alternate file name with lowercase extension.
		 * Both need to be tested for uniqueness as the extension will be changed to lowercase
		 * for better compatibility with different filesystems. Fixes an inconsistency in WP  $alt_filename ) {
						$alt_filenames[ $alt_ext ] = str_replace(
							array( "-{$number}{$alt_ext}", "{$number}{$alt_ext}" ),
							"-{$new_number}{$alt_ext}",
							$alt_filename
						);
					}

					/*
					 * Also update the $number in (the output) $filename.
					 * If the extension was uppercase it was already replaced with the lowercase version.
					 */
					$filename = str_replace(
						array( "-{$number}{$lc_ext}", "{$number}{$lc_ext}" ),
						"-{$new_number}{$lc_ext}",
						$filename
					);

					$number = $new_number;
					$i++;
				}
			}
		}
	}

	/**
	 * Filters the result when generating a unique file name.
	 *
	 * @since 4.5.0
	 * @since 5.8.1 The `$alt_filenames` and `$number` parameters were added.
	 *
	 * @param string        $filename                 Unique file name.
	 * @param string        $ext                      File extension. Example: ".png".
	 * @param string        $dir                      Directory path.
	 * @param callable|null $unique_filename_callback Callback function that generates the unique file name.
	 * @param string[]      $alt_filenames            Array of alternate file names that were checked for collisions.
	 * @param int|string    $number                   The highest number that was used to make the file name unique
	 *                                                or an empty string if unused.
	 */
	return apply_filters( 'wp_unique_filename', $filename, $ext, $dir, $unique_filename_callback, $alt_filenames, $number );
}