http_build_query ignores the key if the value is an empty array. How is this not a bug?

I ran into a problem today where I was passing a key with the value set to an empty array to http_build_query(). E.g.:

$args = array("foo", "bar", array(), "baz");
$qs = http_build_query($args);
echo $qs; // outputs 0=foo&1=bar&3=baz, I expected 0=foo&1=bar&2=&3=baz

This presents a problem for me, since I'm passing some data to an internal API over http and I need to pull all the arguments out on the other side.

Well, I googled this and came up with the following bug report: http://bugs.php.net/bug.php?id=50407 . The terse reply from an admin is, "Not setting is same as setting it empty. No bug."

Can somebody explain to me why this is not a bug? Does anyone have any ideas for a workaround, aside from the lame hack of setting an arbitrary value on one side and interpreting that value as an empty value on the other?

EDIT

Here's why I think it's a bug:

$args = array("foo", "bar", array(), "baz");
$qs = http_build_query($args);
parse_str($qs, $query);
echo ($args == $query); // false, I expect it to be true

I guess perhaps it's naive of me to consider parse_str() and http_build_query() to be inverses of one another.

I'm posting my current "lame hack" solution as an answer below.

I've reimplemented http_build_query to leave empty objects/arrays in the returned query string (suffixed by an '=' symbol). I've enhanced it a bit from the default functionality as well, so all-in-all:

• Maintains empty objects and arrays
• Changed the default enc_type to RFC3986 (relevant to the ages)
• Added a key-value separator argument (ability to override the default '=')
• Removes numeric indices for numerically-indexed key-value pairs

I have not tested this in a production environment (no idea about performance or bugs), and it is not optimized, but very well spelled out.

function buildQuery($input,$numeric_prefix='',
        $arg_separator='&',$enc_type=2,
        $keyvalue_separator='=',$prefix='') {
    if ( is_array($input) ) {
        $arr = array();
        foreach ( $input as $key => $value ) {
            $name = $prefix;
            if ( strlen($prefix) ) {
                $name .= '[';
                if ( !is_numeric($key) ) {
                    $name .= $key;
                }
                $name .= ']';
            } else {
                if ( is_numeric($key) ) {
                    $name .= $numeric_prefix;
                }
                $name .= $key;
            }
            if ( (is_array($value) || is_object($value)) && count($value) ) {
                $arr[] = buildQuery($value,$numeric_prefix,
                        $arg_separator,$enc_type,
                        $keyvalue_separator,$name);
            } else {
                if ( $enc_type === 2 ) {
                    $arr[] = rawurlencode($name)
                        .$keyvalue_separator
                        .rawurlencode($value?:'');
                } else {
                    $arr[] = urlencode($name)
                        .$keyvalue_separator
                        .urlencode($value?:'');
                }
            }
        }
        return implode($arg_separator,$arr);
    } else {
        if ( $enc_type === 2 ) {
            return rawurlencode($input);
        } else {
            return urlencode($input);
        }
    }
}

Example:

$arr = array(
        'hello' => 'world',
        'colors' => array('red','green','blue'),
        'emptyArr' => array(),
        'nested' => array(
            'empty' => array(),
            'fruits' => array('orange','banana','apple'),
            'curly' => 'sue',
            'assoc' => array('a'=>'alpha','b'=>'bravo','c'=>'charlie')
        )
    );

echo buildQuery($arr);

Outputs: hello=world&colors%5B%5D=red&colors%5B%5D=green&colors%5B%5D=blue&emptyArr=&nested%5Bempty%5D=&nested%5Bfruits%5D%5B%5D=orange&nested%5Bfruits%5D%5B%5D=banana&nested%5Bfruits%5D%5B%5D=apple&nested%5Bcurly%5D=sue&nested%5Bassoc%5D%5Ba%5D=alpha&nested%5Bassoc%5D%5Bb%5D=bravo&nested%5Bassoc%5D%5Bc%5D=charlie I hope this finds someone well.

Can somebody explain to me why this is not a bug?

Technically, I don't think it should be labeled a bug. Rather, it's just how they designed the function to behave, whether others disagree with that decision or not.

Your API could just check with if (empty($_POST['2']))

This is my current "lame hack" solution. Note I had to account for the possibility of nested arrays, so my example original array is slightly different from what I posted in the question:

$args = array("foo", "bar", array("red", "blue", array(), "green"), "baz");
$original_array = $args; // save it to compare later
function replace_empty_array_with_fake_string(&$value, $key) {
    if (is_array($value)) {
        if (empty($value)) {
            $value = 'array()';
        } else {
            array_walk($value, 'replace_empty_array_with_fake_string');
        }

    }
}
array_walk($args, 'replace_empty_array_with_fake_string');
$qs = http_build_query($args);

// convert the query string back to an array, this would happen on the "other side"
parse_str($qs, $query);
function replace_fake_string_with_empty_array(&$value, $key) {
    if ($value == 'array()') {
        $value = array();
    }
    if (is_array($value)) {
        array_walk($value, 'replace_fake_string_with_empty_array');
    }
}
array_walk($query, 'replace_fake_string_with_empty_array');
echo ($original_array == $query); // true

Presumably I could come up with a more arbitrary string than "array()" to use as the placeholder.

Lame, I know.

You can simply walk the query params, if empty array, use "[]" instead, like this:

function walkCriteria(&$criteria) {
    array_walk($criteria, function (&$val) {
        if ($val === []) {
            $val = "[]";
        } else if (is_array($val)) {
            walkCriteria($val);
        }
    });
}

Don't use array_walk_recursive. Because it will walk into the empty array and do nothing.

Building on @anyx solution: If you want to preserve the original array and also handle NULL values, you can use this version:

function empty2blank(array $arr) {
 array_walk($arr, function(&$val, $key) {
  if (empty($val)) { 
   $val = is_array($val) ? '[]' : '';
  } elseif (is_array($val)) {
   $val = empty2blank($val);
  }
 });
 return $arr;
}