6.4.1. Problem

6.4.2. Solution

function wrap_html_tag(&$string, $tag = 'b') {
    $string = "<$tag>$string</$tag>";
}

Now there's no need to return the string because the original is modified in-place.

6.4.3. Discussion

Passing a variable to a function by reference allows you to avoid the work of returning the variable and assigning the return value to the original variable. It is also useful when you want a function to return a boolean success value of true or false, but you still want to modify argument values with the function.

You can't switch between passing a parameter by value or reference; it's either one or the other. In other words, there's no way to tell PHP to optionally treat the variable as a reference or as a value.

Also, if a parameter is declared to accept a value by reference, you can't pass a constant string (or number, etc.), or PHP will die with a fatal error.

6.4.4. See Also