PHP: How to create a function to generate string with defined max and min length?

I want to create a self-defined string with defined length of the string. It appears the PHP itself isn't providing me the function I desire

so the function will go with like this:

generate_string("01", "001");

it will show me:

01

when I refresh the page, it'll show me:

001

so on and so forth

And when I loop it:

    for ($i = 0; $i <= 5; $i++) {
       echo generate_string("01", "001");
    }

It's gonna be:

01 001 001 01 01

How do I achieve this?