Basic method

$query = parse_url($url, PHP_URL_QUERY);

// Returns a string if the URL has parameters or NULL if not
if ($query) {
    $url .= '&category=1';
} else {
    $url .= '?category=1';
}

More advanced

$url = 'http://example.com/search?keyword=test&category=1&tags[]=fun&tags[]=great';

$url_parts = parse_url($url);
// If URL doesn't have a query string.
if (isset($url_parts['query'])) { // Avoid 'Undefined index: query'
    parse_str($url_parts['query'], $params);
} else {
    $params = array();
}

$params['category'] = 2;     // Overwrite if exists
$params['tags'][] = 'cool';  // Allows multiple values

// Note that this will url_encode all values
$url_parts['query'] = http_build_query($params);

// If you have pecl_http
echo http_build_url($url_parts);

// If not
echo $url_parts['scheme'] . '://' . $url_parts['host'] . $url_parts['path'] . '?' . $url_parts['query'];

You should put this in a function at least, if not a class.

Here’s a shorter version of the accepted answer:

$url .= (parse_url($url, PHP_URL_QUERY) ? '&' : '?') . 'category=action';

Edit: as discussed in the accepted answer, this is flawed in that it doesn’t check to see if category already exists. A better solution would be to treat the $_GET for what it is – an array – and use functions like in_array().

$data = array('foo'=>'bar',
              'baz'=>'boom',
              'cow'=>'milk',
              'php'=>'hypertext processor');

$queryString =  http_build_query($data);
//$queryString = foo=bar&baz=boom&cow=milk&php=hypertext+processor

echo 'http://domain.com?'.$queryString;
//output: http://domain.com?foo=bar&baz=boom&cow=milk&php=hypertext+processor

Use strpos to detect a ?. Since ? can only appear in the URL at the beginning of a query string, you know if its there get params already exist and you need to add params using &

function addGetParamToUrl(&$url, $varName, $value)
{
    // is there already an ?
    if (strpos($url, "?"))
    {
        $url .= "&" . $varName . "=" . $value; 
    }
    else
    {
        $url .= "?" . $varName . "=" . $value;
    }
}

This function overwrites an existing argument

function addToURL( $key, $value, $url) {
    $info = parse_url( $url );
    parse_str( $info['query'], $query );
    return $info['scheme'] . '://' . $info['host'] . $info['path'] . '?' . http_build_query( $query ? array_merge( $query, array($key => $value ) ) : array( $key => $value ) );
}

Example with updating existent parameters.

Also url_encode used, and possibility to don’t specify parameter value

    <?
    /**
     * Add parameter to URL
     * @param string $url
     * @param string $key
     * @param string $value
     * @return string result URL
     */
    function addToUrl($url, $key, $value = null) {
        $query = parse_url($url, PHP_URL_QUERY);
        if ($query) {
            parse_str($query, $queryParams);
            $queryParams[$key] = $value;
            $url = str_replace("?$query", '?' . http_build_query($queryParams), $url);
        } else {
            $url .= '?' . urlencode($key) . '=' . urlencode($value);
        }
        return $url;
    }

One-liner:

$url .= (strpos($url, '?') ? '&' : '?') . http_build_query($additionalParams);

using http_build_query is recommended because it encodes special characters (for example spaces or @ in email addresses)

Improved version for 2022

This allows existing parameters to be replaced, and also preserves existing URL fragment (the part after # at the end of an URL)

function addParametersToUrl(string $url, array $newParams): string
{
    $url = parse_url($url);
    parse_str($url['query'] ?? '', $existingParams);

    $newQuery = array_merge($existingParams, $newParams);

    $newUrl = $url['scheme'] . '://' . $url['host'] . ($url['path'] ?? '');
    if ($newQuery) {
        $newUrl .= '?' . http_build_query($newQuery);
    }

    if (isset($url['fragment'])) {
        $newUrl .= '#' . $url['fragment'];
    }

    return $newUrl;
}

Testing:

$newParams = [
    'newKey' => 'newValue',
    'existingKey' => 'new',
];

echo addParametersToUrl('https://www.example.com', $newParams);
// https://www.example.com?newKey=newValue&existingKey=new
echo addParametersToUrl('https://www.example.com/', $newParams);
// https://www.example.com/dir/?newKey=newValue&existingKey=new
echo addParametersToUrl('https://www.example.com/dir/', $newParams);
// https://www.example.com/dir/?newKey=newValue&existingKey=new
echo addParametersToUrl('https://www.example.com/dir/file?foo=bar', $newParams);
// https://www.example.com/dir/file?foo=bar&newKey=newValue&existingKey=new
echo addParametersToUrl('https://www.example.com/dir/file?foo=bar&existingKey=old', $newParams);
// https://www.example.com/dir/file?foo=bar&existingKey=new&newKey=newValue
echo addParametersToUrl('https://www.example.com/dir/file?foo=bar&existingKey=old#hash', $newParams);
// https://www.example.com/dir/file?foo=bar&existingKey=new&newKey=newValue#hash
echo addParametersToUrl('https://www.example.com/dir/file#hash', $newParams);
// https://www.example.com/dir/file?newKey=newValue&existingKey=new#hash
echo addParametersToUrl('https://www.example.com/dir/file?foo=bar#hash', $newParams);
// https://www.example.com/dir/file?foo=bar&newKey=newValue&existingKey=new#hash

<?php
$url1 = '/test?a=4&b=3';
$url2 = 'www.baidu.com/test?a=4&b=3&try_count=1';
$url3 = 'http://www.baidu.com/test?a=4&b=3&try_count=2';
$url4 = '/test';
function add_or_update_params($url,$key,$value){
    $a = parse_url($url);
    $query = $a['query'] ? $a['query'] : '';
    parse_str($query,$params);
    $params[$key] = $value;
    $query = http_build_query($params);
    $result = '';
    if($a['scheme']){
        $result .= $a['scheme'] . ':';
    }
    if($a['host']){
        $result .= '//' . $a['host'];
    }
    if($a['path']){
        $result .=  $a['path'];
    }
    if($query){
        $result .=  '?' . $query;
    }
    return $result;
}
echo add_or_update_params($url1,'try_count',1);
echo "n";
echo add_or_update_params($url2,'try_count',2);
echo "n";
echo add_or_update_params($url3,'try_count',3);
echo "n";
echo add_or_update_params($url4,'try_count',4);
echo "n";

 /**
 * @example addParamToUrl('/path/to/?find=1', array('find' => array('search', 2), 'FILTER' => 'STATUS'))
 * @example addParamToUrl('//example.com/path/to/?find=1', array('find' => array('search', 2), 'FILTER' => 'STATUS'))
 * @example addParamToUrl('https://example.com/path/to/?find=1&FILTER=Y', array('find' => array('search', 2), 'FILTER' => 'STATUS'))
 *
 * @param       $url string url
 * @param array $addParams
 *
 * @return string
 */
function addParamToUrl($url, array $addParams) {
  if (!is_array($addParams)) {
    return $url;
  }

  $info = parse_url($url);

  $query = array();

  if ($info['query']) {
    parse_str($info['query'], $query);
  }

  if (!is_array($query)) {
    $query = array();
  }

  $params = array_merge($query, $addParams);

  $result = '';

  if ($info['scheme']) {
    $result .= $info['scheme'] . ':';
  }

  if ($info['host']) {
    $result .= '//' . $info['host'];
  }

  if ($info['path']) {
    $result .= $info['path'];
  }

  if ($params) {
    $result .= '?' . http_build_query($params);
  }

  return $result;
}

$parameters = array();

foreach ($get as $key => $value)
{
     $parameters[] = $key.'='.$value;
}

$url = 'http://example.com/movies?'.implode('&', $parameters);

I think you should do it something like this.

class myURL {
    protected $baseURL, $requestParameters;

    public function __construct ($newURL) {
        $this->baseurl = $newURL;
        $this->requestParameters = array();
    }

    public function addParameter ($parameter) {
        $this->requestParameters[] = $parameter;
    }

    public function __toString () {
        return $this->baseurl.
               ( count($this->requestParameters) ?
                 '?'.implode('&', $this->requestParameters) :
                 ''
                 );
    }
}

$url1 = new myURL ('http://www.acme.com');
$url2 = new myURL ('http://www.acme.com');
$url2->addParameter('sort=popular');
$url2->addParameter('category=action');
$url1->addParameter('category=action');

echo $url1."n".$url2;

After searching for many resources/answers on this topic, I decided to code my own. Based on @TaylorOtwell’s answer here, this is how I process incoming $_GET request and modify/manipulate each element.

Assuming the url is: http://domain.com/category/page.php?a=b&x=y
And I want only one parameter for sorting: either ?desc=column_name or ?asc=column_name. This way, single url parameter is enough to sort and order simultaneously. So the URL will be http://domain.com/category/page.php?a=b&x=y&desc=column_name on first click of the associated table header row.

Then I have table row headings that I want to sort DESC on my first click, and ASC on the second click of the same heading. (Each first click should “ORDER BY column DESC” first) And if there is no sorting, it will sort by “date then id” by default.

You may improve it further, like you may add cleaning/filtering functions to each $_GET component but the below structure lays the foundation.

foreach ($_GET AS $KEY => $VALUE){
    if ($KEY == 'desc'){
        $SORT = $VALUE;
        $ORDER = "ORDER BY $VALUE DESC";
        $URL_ORDER = $URL_ORDER . "&asc=$VALUE";
    } elseif ($KEY == 'asc'){
        $SORT = $VALUE;
        $ORDER = "ORDER BY $VALUE ASC";
        $URL_ORDER = $URL_ORDER . "&desc=$VALUE";
    } else {
        $URL_ORDER .= "&$KEY=$VALUE";
        $URL .= "&$KEY=$VALUE";
    }
}
if (!$ORDER){$ORDER = 'ORDER BY date DESC, id DESC';}
if ($URL_ORDER){$URL_ORDER = $_SERVER[SCRIPT_URL] . '?' . trim($URL_ORDER, '&');}
if ($URL){$URL = $_SERVER[SCRIPT_URL] . '?' . trim($URL, '&');}

(You may use $_SERVER[SCRIPT_URI] for full URL beginning with http://domain.com)

Then I use resulting $ORDER I get above, in the MySQL query:

"SELECT * FROM table WHERE limiter = 'any' $ORDER";

Now the function to look at the URL if there is a previous sorting and add sorting (and ordering) parameter to URL with “?” or “&” according to the sequence:

        function sort_order ($_SORT){
            global $SORT, $URL_ORDER, $URL;
            if ($SORT == $_SORT){
                return $URL_ORDER;
            } else {
                if (strpos($URL, '?') !== false){
                    return "$URL&desc=$_SORT";
                } else {                        
                    return "$URL?desc=$_SORT";
                }
            }
        }

Finally, the table row header to use the function:

        echo "<th><a href='".sort_order('id')."'>ID</a></th>";

Summary: this will read the URL, modify each of the $_GET components and make the final URL with parameters of your choice with the correct form of usage of “?” and “&”

 public function addGetParamToUrl($url, $params)
{
    foreach ($params as $param) {
         if (strpos($url, "?"))
        {
            $url .= "&" .http_build_query($param); 
        }
        else
        {
            $url .= "?" .http_build_query($param); 
        }
    }
    return $url;
}

another improved function version. Mix of existing answers with small improvements (port support) and bugfixes (checking keys properly).

/**
 * @param string $url original url to modify - can be relative, partial etc
 * @param array $paramsOverride associative array, can be empty
 * @return string modified url
 */
protected function overrideUrlQueryParams($url, $paramsOverride){
    if (!is_array($paramsOverride)){
        return $url;
    }

    $url_parts = parse_url($url);

    if (isset($url_parts['query'])) {
        parse_str($url_parts['query'], $params);
    } else {
        $params = [];
    }

    $params = array_merge($params, $paramsOverride);

    $res = '';

    if(isset($url_parts['scheme'])) {
        $res .= $url_parts['scheme'] . ':';
    }

    if(isset($url_parts['host'])) {
        $res .= '//' . $url_parts['host'];
    }

    if(isset($url_parts['port'])) {
        $res .= ':' . $url_parts['port'];
    }

    if (isset($url_parts['path'])) {
        $res .= $url_parts['path'];
    }

    if (count($params) > 0) {
        $res .= '?' . http_build_query($params);
    }

    return $res;
}

Try this function to add URL parameters.

Then you can disable the link when parameter is set so there is no url parameter duplicate.

<?php
  function addQueryString($a)
                {
             if (empty($_SERVER['QUERY_STRING']))
               return '?' . $a;
             else if (!empty($_SERVER['QUERY_STRING']))
              return '?' . $_SERVER['QUERY_STRING'] . '&' . $a;
                }
?>

 <a href="<?php echo addQueryString('lang=en'); ?>">test</a>
 <a href="<?php echo addQueryString('category=5'); ?>">sat</a>

In case you are using WordPress you can simply use

    add_query_args(['sort' => 'asc'], 'http:/example.com/?search=news')

Docs https://developer.wordpress.org/reference/functions/add_query_arg/

Solution for Laravel

If you are using Laravel you can use a function off of the Request object.

// In a controller function
function foo(Request $request) {
    // will be www.laravel.test?hi=there
    $urlWithNewQueryParam = $request->fullUrlWithQuery(['hi' => 'there']);
}

// Outside of a controller function (like in a blade)
request()->fullUrlWithQuery(['hi' => 'there']); // will be www.laravel.test?hi=there

This will also correctly add & and overwrite query params with the same name if they already exist.

You can read more in the docs here https://laravel.com/docs/10.x/requests#retrieving-the-request-url.

Set and remove url query params using PHP:

<?php
function build_url(array $parts): string {
    $url = '';

    $delimiters = [
        'scheme'   => [
            'value' => '://',
            'after' => true
        ],
        'port'     => ['value' => ':'],
        'query'    => ['value' => '?'],
        'fragment' => ['value' => '#']
    ];

    foreach ($parts as $name => $part) {
        if (isset($delimiters[$name])) {
            if (!isset($delimiters[$name]['after'])) {
                $url .= $delimiters[$name]['value'] . $part;
            } else {
                $url .= $part . $delimiters[$name]['value'];
            }
        } else {
            $url .= $part;
        }
    }

    return $url;
}

function set_query_params(array $values, string $url): string {
    $parts = parse_url($url);
    $params = [];
    if (isset($parts['query'])) {
        parse_str($parts['query'], $params);
    }

    $params = array_merge($params, $values);
    $parts['query'] = http_build_query($params);

    return build_url($parts);
}

function remove_query_params(string | array $name, string $url): string {
    $parts = parse_url($url);
    if (!isset($parts['query'])) {
        throw new Exception('Query not found.');
    }
    parse_str($parts['query'], $params);

    if (is_array($name)) {
        if (empty(array_intersect($name, array_keys($params)))) {
            throw new Exception('Any params not found.');
        }

        foreach ($name as $param) {
            if (!isset($params[$param])) {
                continue;
            }

            unset($params[$param]);
        }
    } elseif (!isset($params[$name])) {
        throw new Exception("Param with name - $name not found.");
    } else {
        unset($params[$name]);
    }

    if (!empty($params)) {
        $parts['query'] = http_build_query($params);
    } else {
        unset($parts['query']);
    }

    return build_url($parts);
}

$url= set_query_params(['a' => 'b', 'c' => 'd', 'e' => 'f'], 'https://example.com');
echo "$urln";  // output - https://example.com?a=b&c=d&e=f
echo remove_query_params(['a', 'c'], $url) . "n"; // output - https://example.com?e=f