Undefined offset in PHP
function getusername($input){
    //parse the result
    preg_match("/<username>(.*)?</username>/", $input, $username);
    return $username[1]; // Line 4
}

Undefined offset on line 4.

function getusername($input){
    preg_match("/<username>(.*)?</username>/", $input, $username);
    if( count( $username ) > 0 ) {
        return $username[1];
    }
    return false;
}

function getusername($input){
  $matches = preg_match("/<username>(.*)?</username>/", $input, $username);
  if ($matches) {
    return $username[1];
  }
  return false;
}