Concatenation a string

I am new to PHP and am used to ASP, what would work on ASP dousn't seem to in PHP

I am developing an application that produces a pdf invoice. I am trying to concatenate a string with the result of a recordset field like so:

$phone = 'PH '.phoneNum($row_rsBusiness['Phone']);

The function phoneNum just adds spaces to a 10 digit number to make it more readable.

function phoneNum($string) {
      $pattern = '/(\d{2})(\d{4})(\d{4})/';
      $repl = '$1 $2 $3';
      echo preg_replace($pattern, $repl, $string );
}

The problem is they don't concatenate, the phone number just prints to the screen.