<?php
    
    $urls = array(
             'http://www.example.com',
             'http://www.example.com/a/',
             'http://www.example.com/a/?q1=one',
             'http://www.example.com/a.html',
             'http://www.example.com/a.html?q1=one'
            );
    
    $query = 'q2=two';
    
    foreach($urls as &$url) {
       $parsedUrl = parse_url($url);
       if ($parsedUrl['path'] == null) {
          $url .= '/';
       }
       $separator = ($parsedUrl['query'] == NULL) ? '?' : '&';
       $url .= $separator . $query;
    }
    
    var_dump($urls);

    array(5) {
  [0]=>
  string(30) "http://www.example.com/?q2=two"
  [1]=>
  string(32) "http://www.example.com/a/?q2=two"
  [2]=>
  string(39) "http://www.example.com/a/?q1=one&q2=two"
  [3]=>
  string(36) "http://www.example.com/a.html?q2=two"
  [4]=>
  &string(43) "http://www.example.com/a.html?q1=one&q2=two"
}