Faker offers a couple of methods that let you replace placeholders in a given 
  string with random characters:

lexify - takes given string and replaces ? with random letters
asciify - takes given string and replaces * with random ascii characters
numerify - takes given string and replaces # with random digits
bothify - combines the lexify and numerify
You could try to use one of them, depending on the requirements you have for 
  that random string you need. asciify uses the largest set of characters as 
  replacement so using that one makes most sense.

The following will give you a random string of 20 ascii characters:

$faker->asciify('********************')