Create directory structure in php

a.php

<?php

/**
 * Creates directories based on the array given
 *
 * @param array $structure
 * @param string $path
 * @return void
 */
function make_dir_tree($structure, $path=__DIR__)
{
	foreach ($structure as $folder => $sub_folder)
	{
		// Folder with subfolders
		if (is_array($sub_folder))
		{
			$new_path = "{$path}/{$folder}";
			if ( ! is_dir($new_path)) mkdir($new_path);
			call_user_func(__FUNCTION__, $sub_folder, $new_path);
		}
		else
		{
			$new_path = "{$path}/{$sub_folder}";
			if ( ! is_dir($new_path)) mkdir($new_path);
		}
	}
}

b.php

<?php

$dir_tree = [
		'OpenSQLManager.app' => [
			'Contents' => [
				'Frameworks',
				'Resources' => [
					'images'
				],
				'MacOS' => [
					'sys' => [
						'common',
						'db' => [
							'classes',
							'drivers' => [
								'mysql',
								'pgsql',
								'sqlite',
								'odbc',
								'firebird'
							]
						],
						'widgets',
						'windows'
					]
				]
			]
		]
	];
	
	make_dir_tree($dir_tree);

call_user_func
Why this is used?and what is -- FUNCTION--?