PHP accessing String vars as Array and changing value

Is it ok to do this please?

for($i=0;$i<strlen($str);$i++)
{
    if(!in_array($str[$i],$arAllowedCharset)){$str[$i]='';}
}
return $str;

It works, but I am unsure if I am "allowed" to do that, i.e. $str[$i]='';.

Note: $str is a string variable, $arAllowedCharset is an array containing only alphanu开发者_Python百科merical characters and a dash.

I use that to format user-submitted URLs in a custom CMS.

Thank you.

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It would be easier to check and correct the url with a regular expression. For example

$str = preg_replace('#[^a-z0-9-]#i', '', $str);