<?php 
$arr = array('test' => null, 'test2' => 1); 
echo http_build_query($arr); 
?>

will produce:

If you need to change the enc_type, use this:

http_build_query($query, null, '&', PHP_QUERY_RFC3986);

Or possibly this:

http_build_query($query, null, ini_get('arg_separator.output'), PHP_QUERY_RFC3986);

But not this:

But if you post to a tomcat java server or something else the &amp; might not be handled properly.

To overcome this specify:

http_build_query($array, '', '&');

and NOT

files[0]=1&files[1]=2&...

To do it like this:

files[]=1&files[]=2&...

Do this:

<code>
function cr_post($a,$b='',$c=0)
        {
            if (!is_array($a)) return false;
            foreach ((array)$a as $k=>$v)
            {
                if ($c)
                {
                    if( is_numeric($k) )
                        $k=$b."[]";
                    else
                        $k=$b."[$k]";
                }
                else
                {   if (is_int($k))
                        $k=$b.$k;
                }

$post_data = array('name'=>'miller', 'address'=>array('address_lines'=>array()), 'age'=>23);
echo http_build_query($post_data);

In my case, I simply wanted to pass on the received $_POST data to a CURL's POST data, which requires it to be in the URL format.  If something like a space [ ] goes into the http_build_query, it comes out as a +. If you're then sending this off for POST again, you won't get the expected result.  This is good for GET but not POST.

Instead you can make your own simple function if you simply want to pass along the data:

<?php 
$post_url = ''; 
foreach ($_POST AS $key=>$value) 
    $post_url .= $key.'='.$value.'&'; 
$post_url = rtrim($post_url, '&'); 
?>

You can then use this to pass along POST data in CURL.

<?php 
    $ch = curl_init($some_url); 
    curl_setopt($ch, CURLOPT_POST, true); 
    curl_setopt($ch, CURLOPT_POSTFIELDS, $post_url); 
    curl_exec($ch); 
?>

& as a separator is the URL encoding.
&amp; is HTML encoding.

<?php function http_parse_query($Query) {

// mimic the behavior of $_GET, see also RFC 1738 and 3986.
$Delimiter = ini_get('arg_separator.input');
$Params    = array();

foreach (explode($Delimiter, $Query) as $NameValue) {
    preg_match(
        '/^(?P<name>[^=\[]*)(?P<indices_present>\[(?P<indices>[^\]]*(\]\[[^\]]*)*)\]?)?(?P<value_present>=(?P<value>.*))?$/', 
        $NameValue, 
        $NameValueParts
    );
    
    if (!empty($NameValueParts)) {
        $Param =& $Params[$NameValueParts['name']];
        
        if (!empty($NameValueParts['indices_present'])) {
            $Indices = explode('][', $NameValueParts['indices']);
            
            foreach ($Indices as $Index) {
                if (!is_array($Param)) {
                    $Param = array();
                }
                
                if ($Index === '') {
                    $Param[] = array();
                    end($Param);
                    $Param =& $Param[key($Param)];
                } else {
                    if (ctype_digit($Index)) { $Index  = (int) $Index;  }
                    
                    if (!array_key_exists($Index, $Param)) {
                        $Param[$Index] = array();
                    }
                    $Param =& $Param[$Index];
                }
            }
        }

if (!empty($NameValueParts['value_present'])) {
            $Param = urldecode($NameValueParts['value']);
        } else {
            $Param = '';
        }
    }
}

return $Params;

<CODE>
$a1 = array('x[y]' => array('a'=>1));
$a2 = array('x' => array('y' => array('a'=>1)));
$q1 = http_build_query($a1);
$q2 = http_build_query($a2);
var_dump($a1);
echo '<BR>';
var_dump($a2);
echo '<BR>';
echo $q1;
echo '<BR>';
echo $q2;
echo '<BR>';
</CODE>

A workaround for this is to replace the null values with an empty string:

$data=array(
        'a'=>'apple',
        'b'=>2,
        'c'=>null,
        'd'=>'…',
    );

//    Compensate for fact that http_build_query omits null values
        foreach($data as &$datum) if($datum===null) $datum='';

Losing the null-ness of the original is no real loss if it’s supposed to be a real query string. If the null is important, you could use a dummy value instead.

<?php
$arr = ['foo' => false];
echo http_build_query($arr);
?>

will produce:

The reason is this function will not include null values in the result string:

$data = array(
            'arg' => array(
                null,
                2,
                3
            )
        );
        echo http_build_query($data);

files[]=1&files[]=2&...

but function makes like this

files[0]=1&files[1]=2&...

Here is normal function:

<?php
    $array = array(
        "type" => "welcome",
        "message" => "Hello World!"
    );
    echo urldecode(http_build_query($array, '', ';'));
?>