1. You could create a few explanatory local variables:

    $lettersNumbersSpacesHypens = '/[^\-\sa-zA-Z0-9]+/';
    $spacesAndDuplicateHyphens = '/[\-\s]+/';
   

These would eliminate the comments. (I haven't checked the regexps, other names might be more appropriate.)

Reference: Chapter 6. Composing Methods, Introduce Explaining Variable in Refactoring: Improving the Design of Existing Code by Martin Fowler:

Put the result of the expression, or parts of the expression, in a temporary variable with a name that explains the purpose.

And Clean Code by Robert C. Martin, G19: Use Explanatory Variables.

1. slug could be renamed to cleanSlug (contains a verb) to describe what the function does.

2. I'd rename the $string variable to $slug It would be more descriptive, it would express the intent of the variable.

3. I'd use a whitelist instead of the blacklist. Defining the allowed characters (A-Z, 0-9 etc) would create proper URLs from URLs which contain special characters like é or ű.

1. You could create a few explanatory local variables:

    $lettersNumbersSpacesHypens = '/[^\-\sa-zA-Z0-9]+/';
    $spacesAndDuplicateHyphens = '/[\-\s]+/';
   

Usage:

    $lettersNumbersSpacesHypens = '/[^\-\sa-zA-Z0-9]+/';
    $slug = preg_replace($lettersNumbersSpacesHypens, '', mb_strtolower($slug));

    $spacesAndDuplicateHyphens = '/[\-\s]+/';
    $slug = preg_replace($spacesAndDuplicateHyphens, '-', $slug);

These would eliminate the comments. (I haven't checked the regexps, other names might be more appropriate.)

Reference: Chapter 6. Composing Methods, Introduce Explaining Variable in Refactoring: Improving the Design of Existing Code by Martin Fowler:

Put the result of the expression, or parts of the expression, in a temporary variable with a name that explains the purpose.

And Clean Code by Robert C. Martin, G19: Use Explanatory Variables.

1. slug could be renamed to cleanSlug (contains a verb) to describe what the function does.

2. I'd rename the $string variable to $slug It would be more descriptive, it would express the intent of the variable.

3. I'd use a whitelist instead of the blacklist. Defining the allowed characters (A-Z, 0-9 etc) would create proper URLs from URLs which contain special characters like é or ű.