(The clearest) Write a function that can traverse all files and subfolders in a folder.

opendir -- Open directory handle
Description
resource opendir ( string path)
Returns a directory handle that can later be used in closedir(), readdir() and rewinddir() calls.
If path is not a legal directory or the directory cannot be opened due to permission restrictions or file system errors, opendir() returns FALSE and generates a PHP error message of E_WARNING level. You can suppress the output of error messages by adding the "@" symbol in front of opendir().
readdir -- Read an entry from a directory handle
Description
string readdir ( resource dir_handle)
Returns the file name of the next file in the directory. File names are returned in order in the file system.
Please note the style of checking the return value of readdir() in the example below. We explicitly test whether the return values ​​are all equal (same value and type - see Comparison Operators for more information) FALSE, otherwise any directory entry whose name evaluates to FALSE will cause the loop to stop (e.g. a directory named "0" ).
is_dir -- Determine whether the given file name is a directory
Description
bool is_dir ( string filename)
Returns TRUE if the file name exists and is a directory. If filename is a relative path, its relative path is checked against the current working directory.
Note: The result of this function will be cached. See clearstatcache() for details.
Note: This function cannot be used on remote files. The files being checked must be accessed through the server's file system.
scandir -- List the files and directories in the specified path
Description
array scandir ( string directory [, int sorting_order])
Returns an array containing the files and directories in directory. If directory is not a directory, returns the Boolean value FALSE and generates an E_WARNING level error.

By default, the return values ​​are sorted in ascending alphabetical order. If the optional parameter sorting_order is used (set to 1), the order is sorted in descending alphabetical order.

The above introduces (the most obvious) how to write a function that can traverse all files and subfolders in a folder. , including relevant content, I hope it will be helpful to friends who are interested in PHP tutorials.