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if(!is_dir(dirname(__FILE__) . '/'.$bbb[0])){
    mkdir(dirname(__FILE__) . '/'.$bbb[0],0777);
}

$aaa = '502341';//10023049132
$bbb = explode(',',number_format($aaa));
echo $bbb[0];
if(!is_dir(dirname(__FILE__) . '/'.$bbb[0])){
    mkdir(dirname(__FILE__) . '/'.$bbb[0],0777);
}
if(!is_dir(dirname(__FILE__) . '/'.$bbb[0]. '/'.$bbb[1])){
    mkdir(dirname(__FILE__) . '/'.$bbb[0]. '/'.$bbb[1],0777);
}
...//how to check more $bbb[2], $bbb[3] or even more?

$path = __DIR__;

if (! is_writable($path))
    trigger_error("$path is not writeable");

$str = "502341";
$arr = chunk_split($str, 3, "/");
mkdir($path . DIRECTORY_SEPARATOR . $arr, 0777, true);
                                                  ^--------- Recrusive

<?php
$aaa = "502341";
$bbb = explode(',',number_format($aaa));
print count($bbb); // prints the depth of your folder tree
mkdir(implode("/",$bbb), 0644, true); // creates recursive folder
?>