String sanitisation function

I have a search function on my website. In addition, elsewhere I have a place where users can submit categories for their posts. I want the categories to have an uppercase first letter and the rest lower case. I know there far more complex ways to secure a user input string, but do you think that as a fairly basic function, this is relatively secure?

function purify_string($string){
       $script_tags = "/\+/i";
       $string = strip_tags(addslashes($string));   
       $string = preg_replace($script_tags, '', $string);
       $string = trim($string); 
       $string = strtolower($string);
       $string = ucfirst($string);     
       return $string;
     }

Instead of trying to purify the category for the user, which may or may not have unexpected results, you should simply reject everything that does not conform to your level of standard, and let the user fix it:

function isValidCategoryName($string) {
    return preg_match("/^[A-Z][a-z0-9 ]+$/", $string);
}

If it doesn't match, prompt the user with an error message and tell him to try again. Your job isn't to try to extract a good category from a bad one.

At most you could just correct the case with ucfirst if you want.

StackExchange Code Review Q#54382, answer score: 4