(PHP 4, PHP 5, PHP 7)
is_dir — Tells whether the filename is a directory
$filename
   )Tells whether the given filename is a directory.
filename
       Path to the file. If filename is a relative
       filename, it will be checked relative to the current working
       directory. If filename is a symbolic or hard link 
       then the link will be resolved and checked. If you have enabled safe mode,
       or open_basedir further
       restrictions may apply.
      
   Returns TRUE if the filename exists and is a directory, FALSE
   otherwise.
  
Example #1 is_dir() example
<?php
var_dump(is_dir('a_file.txt'));
var_dump(is_dir('bogus_dir/abc'));
var_dump(is_dir('..')); //one dir up
?>
The above example will output:
bool(false) bool(false) bool(true)
Upon failure, an E_WARNING is emitted.
Note: The results of this function are cached. See clearstatcache() for more details.
As of PHP 5.0.0, this function can also be used with some URL wrappers. Refer to Supported Protocols and Wrappers to determine which wrappers support stat() family of functionality.