如何从字符串中删除点点斜杠？

I want to remove all dot dot slashes in the URL string so user doesn't have access to parent level directory. What I could have is ../../../file, is the below approach safe to use?

$str = '../../../file';
$str = str_replace('..','', ltrim($str,'/'));

EDIT: Thanks for your suggestions and answers, but I also like to know why NOT to use this code? Is it not safe enough? Can it be exploited?

$str = '../../../file';
$str = str_replace('../','', $str);
echo $str;

You can use preg_replace like this:

$string = '../../../file';
echo preg_replace("/(\.\.\/)/","", $string);

I'm not clear with your question, I suspect you need to rewrite your URLs. if it so you can rewrite it by using .htaccess file

RewriteEngine On
RewriteRule ^$ /
RewriteCond %{REQUEST_FILENAME} !-f 
RewriteCond %{REQUEST_FILENAME} !-d 
RewriteRule ^(.*)$ /$1

Or you just want to remove sub-folders from $str alone you just use the below code

$str = preg_replace('/..\//', '',  $str); 

or

$str = str_replace('../','', $str);

We can also do using multiple character select :

echo str_replace(array('..', '/'), ' ', $string);